Sep 2015

Signals II: Block Systems

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.

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.