There are several options I see to do this without worrying about radio interference, each with varying other support components and costs that make them better or worse for some things.
Using TTL to BlueTooth is simple and effective if you plan to use a uP anyway, but comes with about a $3.50 price tag per BT module, and may be cumbersome to pair for users.
The nRF24l01+ is an insanely powerful 99 cent 2.4 GHz transceiver but requires complex coding to make it work with more than 6 units in range of each other. Though, it might not actually be that bad just to have 2 units talking to each other that are hard coded only to speak to each other.
What I think might be the best option is the ESP8266. These cost about $1.85 each, and work over Wifi. Modules can be clients or a server, and they have 2 GPIO pins built in which is probably enough for the simple function reading an input and turning something on on the other end. The only real down side of these is that they run on 3.3v logic rather than 5v and the GPIO pins are not 5v tolerant.
On the question brought up of controlling devices from the cab1/2, there are much better ways to do this than using the switch controller or what not. About $25 in parts plus a receiving transmitter for each device to be controlled would allow for control of as many devices as you like, by reading the raw TMCC serial data. In dumbed down terms, you could use any of the transceiver options above to replace the link between track signal and r2lc.
