What's my best option for a sensor in this situation?

[Chagrin]

What about something like this? Infrared emitter and detector in one package. This is just one example, I'm sure there are dozens of similar devices. It may be necessary to put some reflective tape or something on the bottom of the train if that is permissible. Or maybe it would work without it. At any rate, some experimentation would be needed.

You can blink the IR emitter and watch the change in value from the IR detector. If your detector is seeing ambient light then there will be a marginal change in reading between when the IR emitter is on or off, and if there is something reflecting the IR light then the change in reading will be much more pronounced.