Possibly you could use a Lionel ARC, and trigger start and stop sequences when a particular track sensor is occupied or cleared.
The problem is knowing which train is to receive the command. I don't think there's any way of doing that with the ARC.
I have implemented a full microcontroller automated train running system for TMCC and DCS, but it requires RFID tags in locomotives or rolling stock to tell the system which train is in which block.