For a design I did for a client, I used the reed switches and magnets suggested above. Another thought if you needed continuosly variable sensing might be to have something roll alongside the train with an encoder wheel. Then you could sense the actual position anywhere on the track (dead-reckoning). You could probably even incorporate that into the train wheels directly. Obviously more complicated than the reed switches and magnets and would require modifying your train.
And last possible suggestion would be just to install limit switches along the track that your train bumps as it goes. But I bet that's similar to the H0 Maerklin system with feedback modules mentioned by Shpaget.
If you mean passing control signals along the track itself... hmm... I can think of a few rather complicated ways to do that like introducing high-frequency ripple or AC coupled/decoupled RF, but the simplest approach would just be using wireless.