Signals II: Block Systems


This is the second in a series on lineside railroad signals. Today’s post is going to cover block signals around the world, profiling a number of systems of increasing complexity, leading up to an overview of how Japanese block signals work.

This follows the previous post, which provided an overview and history, but also background information needed for this post to make sense, so you really ought to read that one first. It also included a section at the end with all of the references (plus some comments near the start about other sources of information). A future post (or posts; it’s a complex topic) will cover signals related to interlockings.

Block Signals

On a railroad, lineside signals provide information to the person driving a train (the driver or engineer, depending on which country you live in; I’m going to use the word “operator”). This allows them to go faster than if they were limited to what could be seen directly. Trains are heavy and steel wheels on steel rail slide fairly easily, so it can take more than a mile (1.6 km) to stop a train moving at a reasonable speed.

Braking distance isn’t the only thing that affects train speed. At places where tracks diverge, or when changing tracks, a train may need to slow down due to the speed limit imposed by the turnout(s) being used. For this reason, signals used at places like this (one of several types of “interlockings”) get more complicated. As noted above, I’ll address that aspect in a future post.

Where trains don’t have a choice of direction, what controls speed are two things: unchanging limits imposed by equipment or track, and variable limits due to conditions ahead. Inherent limits are things the operator knows before boarding the train: the limits of the equipment and permanent speed limits imposed by track geometry (sharp turns, etc), and temporary limits (such as a limit imposed until a known problem can be fixed). Those limits may also be posted on signs, although this depends on the railway, and often the operator is required to memorize both the normal limits and any special limits in effect that day.

A green signal, meaning “clear” or “proceed”, doesn’t normally specify a particular speed, but rather tells the operator that the track is clear ahead and can be taken at whatever is “full speed” for this stretch of track.

Block signals historically have worked to limit speed based solely on knowing how many block sections ahead of the train are clear, up to some maximum. The speed limit associated with a given indication is either encoded into the interpretation of the signal (e.g., “yellow means 30 mph”) or another detail the operator needs to memorize. The signals work by using electrical “track circuits”, which can also detect rails that break due to accident or environmental conditions (rails stretch and shrink as temperature changes, and sometimes they snap).

This makes block signals, usually, much simpler than interlocking signals. However, block signals adjacent to an interlocking may be a hybrid of the two, and able to display additional information relevant to the interlocking while still being part of the block. We’ll cover that aspect with interlockings, and today focus only on block signals away from interlockings. These are sometimes called “intermediate block signals”.

Fundamentally block signals provide an indication of the distance (in block sections) a train has before it must come to a halt. That can be “unlimited” (meaning longer than the worst-case braking distance) or some number of blocks. It’s not that simple of course.

Block Design

As mentioned previously, block length is related to braking distance. In particular the train with the worst braking distance from full speed has to have sufficient advance warning that a signal will be red to stop before arriving at it, because passing a red signal means that the train will be in potential conflict with some other train, and a collision could result. The British phrase “Signal Passed at Danger” (abbreviated SPAD) has come into fairly general use when describing such failures, even in the U.S., where a red signal is not described by the label “Danger”.

This advance warning can be provided with a typical three-color system, using yellow to warn of an upcoming red signal. But this assumes that the spacing between signals is longer than that worst-case braking distance. And in some cases, both dense urban environments and mountain railroading with heavy trains, there can be reasons to make blocks shorter than this distance, such as to allow more trains through in a given time period, and this has led to block systems with more than three aspects.

Recap: Aspects and Indications

An “aspect” is the visual form of the signal. It can be an arrangement of colored lights, a single lamp that can change color, a moving wood or metal semaphore arm, or lights oriented to mimic the positions of a semaphore. What these mean is called the “Indication”, and there’s a fairly common vocabulary to the basics: red or a horizontal line generally means stop, yellow or an angled line is a warning, and green or a vertical line indicates that the track is clear ahead for some distance. Actual indications have very specific wording that varies by railroad and may impose multiple restrictions on the operator, such as a yellow light meaning “Immediately reduce speed to medium speed and prepare to stop at the second signal”.

And terminology also varies more broadly: for example yellow means the same thing in both the U.K. and U.S., but in the U.K. it is called “caution”, while as we saw in the previous post in the U.S. it is called “approach”.

But different railroads use even color-lights differently, particularly when they need more than the basic three indications. Even yellow doesn’t always mean “the next signal is red”, although that’s a very common indication.

Related to this are speeds. Not all signaling systems communicate speed directly, but all are fundamentally about speed: is it safe to go “full speed”, whatever that means at this location, or is some lesser speed required, possibly coupled with a requirement to take other precautions? Three typical speeds show up fairly often:

- Restricted speed (25 kph or 15 mph) is the maximum speed a typical train can move at and still be able to stop within the normal range of vision. Often “restricted speed” doesn’t mean a specific rate, but whatever rate the operator determines is the maximum safe rate for stopping, which might be lower at night or in a storm.

- Medium speed (around 50 kph or 30 mph) is the maximum speed a train can take on the diverging route through a normal switch. This too varies by railroad, as some equipment is more sensitive, and some railroads are more cautious. And despite blocks having no switches, it’s often used in conjunction with block signals, although there it is referring to a stopping distance used to calculate minimum block length.

- And Limited speed (around 75 kph or 45 mph) is a typical limit on the diverging route through a high-speed switch, and is also common in some kinds of multiple-block systems as a speed at which a safe stop can be made for a problem a couple of blocks ahead. This is even more variable, and “limited” speed on one railroad may be about the same as “medium” on another.

Different railroads may use other names, or assign different speeds to these labels, but the basic concept is surprisingly common across very different railroads.

One Block (or two), Two Aspects, and optional Distant Signals

Some signals can only show two indications, “Stop” (usually a red light) and “Proceed” (usually a green light). By itself this isn’t very useful. A train moving above about 25 kph (15 mph) won’t have time to stop when it sees a red signal with no advance warning. This is sometimes used on low-speed transit or light rail lines, or within yards or stations (e.g., dwarf signals in North America, and shunting signals pretty much everywhere else, are often two-aspect signals).

One solution for more general use, sometimes used because it’s very simple, is to connect a second signal to the first and use it to warn of the status of the upcoming signal. This is called a “distant” signal. And in the usual case, this signal can only display “Proceed” or a “Caution” indication (usually a yellow light). If the blocks are short, the distant signal for the second block will be mounted to the same mast below the block signal for the first block, which gives the following set of aspects:


Note: in these diagrams I’m using a simplified signal with separate circles showing the color on each head (or in some cases two colors above each other on one head) and a simple aspect using red, yellow and green lights (with rays around a light if it is flashing, an aspect often used to avoid adding another bulb). The signal is placed on the diagram so that the base marks the beginning of the block protected by the signal, with the top extending into that block (i.e., in the direction of travel a train passes the base before the top). Not all railroads use color lights (although many do), but what is really being shown here is the indication signaled by those lights, although in some cases I’m also showing light combinations specific to a given railroad or standard used by more than one railroad.

The way this works with distant signals is that the top head is based on the state of the immediate track block: red if occupied, green if not. The lower head is based on the next block’s top head: yellow if that one is red, green otherwise. There are effectively three aspects with this approach: red-over-green for Stop, green-over-yellow for Caution, and green-over-green for Proceed. In color-light systems this uses four bulbs: red and green on the top head and yellow and green on the bottom.

In traditional British practice using semaphore blades with lights, Stop (or Danger to use their terminology) was “red over yellow”, likely to avoid potential confusion from having green appear on a cautionary signal, but otherwise identical.

While distant signals were often on the mast of the block signal before the one they provided warning for, when blocks are longer, the distant signal can be a separate signal placed some distance ahead of its block signal (the safe braking distance), but well away from the previous block signal.

Either way, this solution is really easy to wire up, but it has a cost: it uses one extra lamp per signal mast compared to the next design. This was more common back in the days of manually-operated signals. If you’re going to do all this just to get an Approach indication, why not simply add that lamp to the first head and make the logic (relays, or whatever) controlling that head a bit more complex? Overall, it’s going to cost less. And that’s what most systems do. Distant signals are sometimes used though, where local conditions require an advance warning. Japanese signaling makes use of them, although not like this.

In North America, two-head signals aren’t this kind, and are used to provide considerably more complex signals.

Two Blocks, Three Aspects

The most basic block signal method in general use today is called the “two-block, three-aspect” system. This is the same one described in the previous post in the discussion of ABS and APB. This was one of the earliest methods in use, going back to the 1800’s. And where trains are infrequent or widely-spaced, it’s still a good system and much more cost-effective than more complex systems on very long lines. It has some inefficiencies in congested urban areas, particularly with higher-speed trains, so it’s not the only system in use, but it’s probably the most common.

One interesting development is that as computers become more tightly interwoven with signaling systems, additional information conveyed by aspects (meaning speed or routing) is being separated out and indicated by another means such as wireless (this actually has roots that go back further in time). And this is causing block signaling to be simplified back to a three-aspect approach, without losing the advantages of multiple advance blocks for braking. I won’t go into that further in this post, and outside of Europe it’s not common yet, but it is happening.

On many railroads the three-aspect system is achieved with a single light of each of three colors (or a light with a moveable lens that can display one of three colors). The yellow signal means that the next signal encountered is, at this instant, red. It may no longer be red when the train gets to it, but it is red now and the train has to proceed under the assumption that it will remain red. The green signal means that the next signal is, at this instant, green or yellow and will not be red when encountered (there are circumstances where this could be violated and the signal actually turn out to be red, and some aspects of block design and train operating rules are specifically to address those situations, but the base assumption is that it can’t be made red ahead of a moving train by any normal event).

I’m not aware of any signaling system that doesn’t allow for a simple three-aspect use, although there are some places where a specific signal may only show red and green or red and yellow. There are often more complex structures allowed, but the basic concept of Stop Here, Stop at Next Signal, and Don’t Stop seems to be universal as a least-common-denominator. In this system, the yellow signal is the first warning of an upcoming red signal, so a train at full speed must be able to brake to a stop in less than the distance between signals. This is described as the Braking Distance being equal to the spacing between signals, although it is really the minimum spacing. The signals can be (and often are) farther apart, and I’m glossing over complexities such as signal overlap (covered in the previous post) and additional space to allow for human reaction times and a safety margin.

This system is also called a “two block” system, because two trains at top speed must be separated by at least two blocks or the following one will be slowed down by yellow signals. This is described as the “headway” between full-speed trains being two blocks in length. Any closer, and the following one won’t be going at full speed. Trains often do follow closer, but the maximum capacity of a line exists when they’re at their headway spacing and moving at top speed.

In the U.S., the AAR was founded in 1934 and standardized the three-aspect system some time before 1942 (the first book I’ve seen describing this is from 1942), but this was really just acknowledging existing behavior on the Pennsylvania Railroad (PRR) and other railroads.

In the AAR standard, separate labels are provided for aspect and indication, as summarized in the following diagram. In addition, the indication for the Approach aspect included a requirement to immediately reduce speed to Medium Speed (which was defined as either 30 MPH or half of maximum speed, depending on the railroad). Restricted Speed didn’t have a numeric designation, instead it was (and often still is) characterized as the speed at which the train could stop short of “a train, obstruction or switch not properly lined” while being on the lookout for broken rail.

As an interesting historical note, the phrase about immediately lowering speed to Medium at an Approach signal was not in the first release of the AAR standard. It was added to prevent operators (“engineers” in the AAR documents) from remaining at maximum speed until they were too close to the Stop signal to brake effectively. It’s not clear from the description I read if this change was made due to an accident or accidents, or merely out of a lack of trust in the judgement of the engineers. My suspicion is that the AAR copied one existing system, but other railroads didn’t trust their engineers to be as careful and wanted a tighter limitation on behavior, so they amended the rules to get more railroads to adopt them.


If you model U.S. prototypes, you’ll notice that almost all commercial signal control systems are designed to model this kind of block system. That’s because it’s so common (and with three lights it’s relatively inexpensive to make the control circuitry). And that’s likely true elsewhere, as every country has some form of it and usually equally simple in implementation, although the appearance of the signals may vary (German practice, for example, uses a distant signal with two yellow lights on a diagonal to indicate that the next signal is red, although this may only apply at interlockings; I’m working off wikipedia here, and it’s less than clear). Kato’s Japanese signals are designed to replicate a three-aspect system, although Tomix’s Japanese signals add two more aspects for more complex lines.

Doubled Aspects

Over time, trains have become longer, heavier and/or faster, and braking characteristics have changed. In some cases the spacing of signals no longer allows enough distance for trains to stop. The usual solution here is to add some kind of preliminary warning, rather than moving all of the signals to a new spacing. More complex signal systems are often the result, as we’ll see shortly, but a simpler fix is just to provide two caution signals in a row.


Here the first Caution indicates a need to begin braking, perhaps for all trains, or perhaps only for those traveling above a set speed or which have other characteristics requiring additional distance to stop. While this kind of enhancement is quite common, and is an easy “quick fix” when the operational characteristics of trains change, it either forces some trains to slow down early, reducing efficiency, or calls for operator judgement to determine if they need to start braking at this Caution or some upcoming one, which leads to human error and decreased safety.

Since safety and efficiency are the twin goals of signaling, both of those outcomes are undesirable, and that has led to the creation of new forms of advance warning, by creating more complex signals that can display additional aspects. That comes at a financial cost, but often the gains in safety and/or efficiency make that worthwhile.

Four Aspects, Three Blocks

If signals need to be closer together than the advance warning of a three-aspect system will allow, but the railroad wants the system to be less prone to human-error failures, that leads to more complex systems. You can potentially have any number of aspects. With each added aspect, blocks can be smaller for the same braking distance. Unfortunately, with complexity comes both added cost, and potentially decreased efficiency. The largest I’m familiar with allows for five blocks between full-speed trains (note: Japanese signals potentially allow for six blocks, but I don’t think any railroad actually does that).

The most common enhancement, and nearly every country has some variation of this, is to insert a new aspect between Proceed (green) and caution (yellow). In the U.S., the PRR had begun using this kind of signal sometime in the 1920’s. And as we’ll see below, the the LMS Railway in the U.K. had experimented with a similar system extending this with another block in 1932. Neither system caught on with other railroads in the short term, although in the U.S. the AAR would standardize a 3-block system by 1942, and eventually it was widely adopted.

Today the added aspect is often referred to as “Advance Approach” (remember that another name for Caution is Approach), and its indication is based on the idea that the signal two blocks ahead is red. In British practice this is called “Preliminary Caution” (or sometimes “Attention”) and indicated by two yellow lights above one another on a signal mast, and this has been copied by many other systems, although not by Japan.

In the AAR standard the advance warning of a yellow signal was called Approach Medium, due to the requirement to have reduced speed to Medium (30 MPH) before passing the Approach signal (which required any train going above Medium to immediately reduce to that speed). As noted above, this was the method the PRR employed, and used on their cab signal system. However, other U.S. railroads that eventually adopted a 3-block, 4-aspect system didn’t follow the AAR standard.

Note: the use of “Attention” for a double-yellow may be historical, deriving from LMS using this word to describe two signal aspects used in advance of yellow. However it appears to be in current use in Indian railway signaling (derived from British practice) and I’ve seen it used a few times to describe British signals, although “Preliminary Caution” seems more common and is how it was described in their 2002 standard (there is no simple name given in the current 2010 version of that standard).


In Japan, Reduced Speed (shown by a yellow light above a green one) is the equivalent of Approach Medium or Preliminary Caution, and the indication typically allows speeds 10 - 20 kph above those allowed by a Caution signal, which is a relatively minor difference, as Caution for passenger trains usually means somewhere between half and three-quarters of maximum speed (see my Signal Aspects page for specific speed ranges from the MLIT document).

When you consider that in North American signaling, a red subordinate head is to be interpreted as absent (the red color conveys no meaning, and the signal is identical to one with one fewer head displaying the same colors), it can be seen that the Japanese aspects are identical to the American ones in color, with the only difference being that Japanese signals put all the lamps on one head and light them selectively. Japan diverges from North American practice in a number of other ways, and this may be pure coincidence.

Even in North America, there is considerable variation. Not all railroads followed the AAR standards, particularly in later years as needs changed. One of the most common variations was the use of a flashing yellow as the advance warning of a yellow. This saves a bulb, making it cheaper, and is a fail-safe aspect, since a failure of the flashing relay converts the signal to a more-restrictive yellow. As we’ll see below, this is also used in France (and likely many other places), presumably for the same reasons. When used by North American railroads, a flashing yellow goes by names such as Advance Approach, Approach Medium, and, in Canada, Advance Clear to Stop. One typical example is the pre-merger Southern Pacific (and its successor the Union Pacific continues to use it under the same name).


According to the description of the AAR standard, a three or more block system is desirable where blocks need to be short because either traffic density is high, where steep descending grades exist (i.e., where heavy trains take longer to brake), where high speed trains are mixed in with normal ones, or where tracks approach an interlocking where trains may be delayed (i.e., several trains may be waiting sequentially outside the interlocking).

One reason three-block systems were introduced was the increasing speed or weight of trains, particularly in the 1930’s. With existing three-aspect signals changed to four-aspect, the existing signal spacing could support higher-speed passenger trains and heavier freight trains, both of which had longer braking distances, without the expense of relocating all of the signals to wider spacing.

Five Aspects, Four Blocks

But it doesn’t end at three. If three blocks are good, then four would be even better, right? Well, maybe. With more blocks comes decreased signal spacing and improved efficiency, but at a fairly high cost in extra signals and the related electronics. Additionally the increased complexity of aspects means that each signal will need more lamps, and more electronics, which only adds to the cost. And then there’s reliability: more parts means more frequent failures; simplicity is a very strong virtue in signal design. There’s clearly a case of diminishing returns here, and while there’s value to having more blocks, a need that justifies this despite the associated problems is likely harder to come by.

The lack of four-block, five-aspect systems on many railroads suggests that this is not a very common need. But they do exist. North America, Japan, France (SNCF) and New South Wales, Australia all define such systems. However, each railroad with such a system seems to have evolved it on their own, with an added aspect unique to them. This is where the commonality of design starts to break down.

In the U.K., the LMS (London, Midland and Scottish) Railway tried such a system around Mirfield junction in 1932, but it was only used on one other line. The Mirfield signals actually lasted until 1970.


France’s SNCF has a four-block system that uses five aspects, both placed between Caution (Avertissement) and Proceed (Voie Libre, literally “free way” or “clear track”). Note that the flashing green aspect isn’t inherently fail-safe, as a failure of the flashing relay will convert the aspect to the less-restrictive Voie Libre unless additional circuitry is used to correct for this.


As noted earlier, France isn’t the only country to use a flashing yellow for the advance warning of a yellow signal.

Japan also uses flashing signals, but differently. Their first warning of a yellow is a yellow-over-green Reduced Speed aspect, and the advance warning for that is a flashing yellow-over-green, called Less Reduced Speed. This is fail-safe, as a relay failure converts Less Reduced Speed to Reduced Speed.


It’s not clear how often this actually gets used, as Reduced Speed seems to often (but not always) be a stand-in for Caution (based on the relationships discussed in the MLIT standards) and is followed by a Stop signal. As we’ll see below, Japan also has a four-block system with a special low-speed aspect, and that one I’ve seen described in a technical paper.

In the U.S., the AAR did not standardize a four-block, five-aspect system (at least not in 1942, and I haven’t found later references to one), although the 1942 standard did mention that one could be created by inserting an additional, undefined, “Limited Speed” aspect before Approach Medium. Later North American railroading did just that, although the inserted signal varied by railroad as to both its aspect and indication.

One example is Amtrak, which uses the Eastern U.S. NORAC signal standard on its high-speed “Northeast Corridor”, with trains operating at up to 150 mph (241 kph) under cab signals. The line is also shared by slower commuter and freight trains using lineside signals that are limited to 79 mph (127 kph) with a Clear aspect. Here Limited means 40 mph (64 kph) for freight and 45 mph (72 kph) for passenger trains.


The NORAC rules are particularly complex because they represent a fusion of several earlier railroads that had differing practice, including the Pennsylvania, New York Central and New Haven Railroads (which were merged by the government to form the Penn Central Railroad in 1968 & 1969 when they went bankrupt). Rather than Approach Medium, the advance warning on an Approach can also be given by an Advance Approach (single flashing yellow) similar to the Western U.S. practice noted earlier, and there are several additional aspects used when approaching an interlocking.

The states making up the former Soviet Union had a formed the Organization for the Collaboration of Railways (Организация Сотрудничества Железных Дорог, abbreviated OSShD, OSSJD or OSŽD in Roman alphabets, and ОСЖД in Cyrillic) to coordinate their systems and resolve conflicts affecting international rail transport. Among other things it developed technical standards including those for railway signaling, and these have spread far beyond the original member countries. China’s signaling system is a variation on this.

The OSShD system is at heart a five-aspect system, but with a fairly complex set of aspects (13, I think) used to convey very precise information about speeds in the next two blocks (the Chinese variation conveys route information instead of speeds). I haven’t managed to puzzle out just how it is used in practice, but there are five speeds involved: Clear, Limited, Medium, Slow and Stop, so it is probably a variation on a four-block system. It’s possible that larger numbers of blocks can be described (it’s equally possible that it’s normally used for just three, and the added aspects are only for interlocking use).

Lower Speed Aspects

Notice that in all of the systems described up to this point (except for OSShD), the Caution aspect (or equivalent) is always followed by a signal showing Stop. But that’s not always the case.

Japan also has a 5-aspect system described as being for a “high-density traffic section”. What makes this different from other 5-aspect systems we’ve looked at, is that it’s essentially a four-aspect system with a fifth aspect between Caution and Stop. The new aspect is described as “Speed Restriction” or “Restricted Speed”, and it imposes a 25 kph (15 mph) limit on train speed. This was probably done so that urban commuter lines could have short blocks, allowing trains to follow each other closely at low speeds rather than forcing a stop, but many of these lines also served freight trains traveling to ports and urban industrial areas, and that may have been a factor as well.


Australia’s state of New South Wales takes a different approach, creating a five block system using only five aspects, by repeating one of them, as well as inserting a Low Speed aspect between Stop and Caution. This is a hybrid between a four-block system optimized for short blocks with low-speed operations, and a doubled-aspect system used to add higher-speed trains to an existing system. It uses two heads, staggered on opposite sides of the mast.

As mentioned earlier, this kind of doubling, in this case the repeated Medium aspect, puts more of a burden on the operator of the train to recognize that they’re seeing the first or second appearance of a Medium aspect, but doing it this way reduces the financial cost of providing the extra warning to allow higher-speed operation. It’s likely that this is a transitional state, and at some point a human-error accident will cause the railroad to re-evaluate the cost of adding a separate “Preliminary Medium” aspect, and this will become a 5-block, 6-aspect system.


North American railroads have also used the doubled aspect technique, either with two sequential Cautions or two sequential Approach Mediums (the North American equivalent of NSW’s Medium). This was done on some three-aspect lines converted to four aspects, but was generally held to be inefficient as it resulted in speed reductions in advance of need, making line utilization lower than it might otherwise have been.

Higher Speed Aspects

While you can address the needs of higher speed trains by inserting aspects between Proceed and Caution, it’s also possible to define a separate “high speed clear” aspect. This is generally done where most trains are limited to a lower speed, but an added aspect is needed for a subset. Functionally this is no different from changing the definition of clear, and allowing most trains to run at their top speed at the first level of cautionary signal. And systems that work this way aren’t very common. But a number of them do exist.

In the U.K., when 140 MPH (225 kph) trains were being tested on the otherwise 125 MPH (200 kph) East Coast Main Line in 1988 a flashing green “High Speed Clear” aspect was introduced, to allow the higher speed. Trains limited to 125 MPH would, of course, go at that speed with either a normal steady green clear or the flashing green. And in the event of a failure of the flashing relay, a high-speed clear signal would fail-safe to an ordinary clear.


Japan, as we’ll see shortly, also has a high speed clear aspect, indicated by two green lights. However, most Japanese lines are narrow-gauge, and stability is an issue at higher speeds for narrow-gauge trains, so this isn’t widely used. In fact, only one narrow-gauge line (and one non-Shinkansen standard-gauge line) use it. Shinkansen likely don’t need this kind of application, as those lines tend to use trains with similar high speeds (there are exceptions, but likely not enough to matter).

The Japanese Block Signal System

In Japan, most non-Shinkansen are limited to 130 kph (81 mph), and many are limited to slower speeds. Two non-JR lines are exceptions, operating trains at speeds up to 160 khp (100 mph): the Hokuetsu Express on the narrow-gauge Hokuhoku line and the Skyliner services on the standard-gauge Keisei Narita Airport line. These lines both use a High-Speed Clear, very similar to the U.K.’s ECML signals except for the different speeds associated (and the fact that lower-speed signals use the normal Japanese aspects).

If you put these all together, the Japanese block system can display seven aspects, and would allow for up to six blocks of headway. It’s unclear if this actually happens anywhere. More likely is that different lines use five-block systems omitting either the High Speed Clear or Speed Restriction aspects, and the four-block system is more typical. It’s even possibly that it’s the largest number of blocks, and the other aspects are for special situations; I couldn’t find anything definitive beyond the four-block system using Restricted Speed, which is documented in an article on Japanese Signals (PDF).


And, of course, four-block systems are going to be even more common than five-block systems. But the Japanese system is flexible enough to deal with six block separations if someone needed to.

I’m not aware of any other railroad that has a system supporting more than five blocks, and NSW was the only other five-block system I could find. For everyone else, four blocks is all the headway that they need.

As we’ll see in a future post, Japan uses the same aspects for interlockings, unlike other railroads that often have a multiplicity of special signals for that purpose. That’s in keeping with Japanese railroading’s overall tendency to favor simple, minimalist solutions, under the overall philosophy that something that isn’t there at all can’t break and cause a problem.

But in closing, I want to note how much commonality there is across railway signaling around the world. The basic three-block system is nearly universal, and extensions to it to make four and five-block systems are very similar (even though the appearance of signals changes, the meaning is usually the same). Even the uncommon optimization for short blocks and low speeds seems to be done similarly, when it needs to be done at all.

I suppose that this shouldn’t be a surprise, given that all of these systems are solving the same basic problems using similar technologies. But I hadn’t anticipated that, and as I dug into this and found the commonalities I was quite surprised.