Just checking in to this thread. Afraid I don't have a lot to offer at this point. I don't have any of the various IR modules to play with right now. In general, I find using an insulated rail to be one of the huge advantages of 3 rail track in the first place, but IR sensors are nice and easy to work with.
I wonder if it might be worth the effort to custom build a PCB for this purpose? It seems the main issue is that the modules are not sensitive enough at any sort of range. I know IR can be made to work here, after all the big manufactures have done it, but I'm unsure if the design of the modules allows for that sort of range.
It may also be worth looking at ultrasonic distance sensors. They are larger than the IR solution, but might give a better result if going for cheap modules? Really not sure, as I've not used these either.
Anyway, I look forward to seeing what you all come up with. Afraid I have too much on my plate right now to really dig into it right now.
JGL
