[qingcharles]

We assumed it was some sort of electrical connection between the two through the wheels of the car. Is that how the trains activate it?

[aidenn0]

That's how it worked 30 years ago at least. There have been upgrades to the signaling system since then, but detecting a circuit being closed is so simple and reliable that it would be odd to do away with it.

[qingcharles]

It definitely surprised us that it worked with the car. At least we know it was accurate in the third BTTF movie when he rolls through the crossing ;)