Since the beginning of DCC we have been able to send information out to locomotive decoders as they run on the layout, data flows out from the command station but, up until now, it has been difficult to receive data back from the locos. DCC is known as a one way protocol.
RailCom allows information to be sent back from the locomotive decoders - for example the address may be read in a monitored section which allows you to positively identify which loco is on that track. Because information can travel to and from the loco decoders, RailCom is known as Bi-directional communication.
All Lenz decoders are now supplied with RailCom as standard. (RailCom is active as part of the factory setting - it may be turned off by setting bit 4 of CV29 to 0) RailCom is now also used by Roco Z21, ESU Ecos and Uhlenbrock MARCo (RailCom Receivers specific to Uhlenbrock LocoNet system).
The following film shows the use of the LRC120 RailCom Display to monitor occupation of a track:
RailCom is now being incorporated into the DCC Language - Technical details may be found under the NMRA Recommended Practices RP 9.3.2. The RPs are the full technical documentation for the DCC Standard - They contain all the information a manufacturer needs to make DCC devices so can be a bit dry for the majority of us - Thankfully I do not need to understand it all to play my part.
RailCom is registered trademark of Lenz Elektronik GmbH