DCC is normally a one-way protocol, sending commands from a command station to a decoder. DCC allows CVs to be read from a train, but there’s no current NMRA standard for how a DCC decoder can report information back to the command station during operation. This is most useful in automated systems, where a block occupancy detector can report both that the block is occupied, and by what train (decoder address). There could be other applications as well, but most people don’t need or use any form of what I’ll call DCC telemetry.
The NMRA would like to have a standard, or at least a Recommended Practice (RP) for this. There’s a draft RP (RP 9.3.2) that describes RailCom as a solution to this problem, but this has apparently been mired in licensing issues or some other manufacturer politics, and hasn’t advanced much in years, although the RP draft was updated recently. Lenz originally developed the technology, and a page on their site says they’ve licensed it to the NMRA, so perhaps this will move forward some more.
Absent a standard, Digitrax developed a proprietary solution they call Transponding, support for which is found on most (all?) of their decoders and occupancy detectors.
Decoders and Telemetry
If you are using telemetry, you need the decoder to support the method you’ve chosen. In general, a decoder only supports one (I don’t know of any that support both), and many don’t support either.
Lenz sells a separate RailCom-only decoder that could be added to a train to add RailCom to another decoder that doesn’t support it. It’s the LRC100 (11mm x 9mm x 1.9mm), and the instructions say it wires only to the pickup rails, so presumably you could add it to a cab car as well as a motor car. At US$17 typical cost, this plus a motor decoder isn’t much cheaper than buying a RailCom-equipped motor decoder, but it would be helpful if you prefer a non-RailCom model, or want to add the capability later.
For telemetry to work, it probably needs a load on the decoder, so a cab-only decoder that only controls small LED lights may not provide usable telemetry, even if the decoder supports the feature when wired to a motor. But I haven’t experimented with that and Lenz’s description of the LRC100 suggests otherwise.
Telemetry is useless without something to listen to it. The signal is going back on the rails, but I’m not aware of any current Command Station that works with either, and at best all it would know is “train X is on the track”. Usually you’ll want to have block occupancy detectors that intercept the telemetry signal, and pass it to a computer or other system. For example, Digitrax’s BDL168, when equipped with RX1 sensors, will issue messages over LocoNet identifying which train address has entered or left a block, and these messages can be received by a computer and used to trigger other actions.
To do all of that, you’ll need the sensor, a way to connect a computer to the control bus over which the information is sent, and software on the computer to do something with it.