A RailCom capable decoder is necessary for use with the RailCom location detection. If you do not wish to or cannot replace an older decoder with a modern ESU V4 decoder then the RailCom transmitter module comes in handy: this small transmitter is installed in addition to the locomotive decoder or individually in a cab driving trailer and is simply soldered to the wheel pickups. Due to its small dimensions of only 11.0 x 9.0mm it fits in almost any locomotive and can then be programmed like any other DCC decoder. Of course, the module supports 2-digit and 4-digit addresses.
With an ESU ECoS you can employ RailCom in conjunction with (older or more recent) Marklin mfx decoders: The ECoS will be able to generate the connection between the RailCom transmitter module and the locomotive decoder.