Any idea how the road barriers in the USA detect a train to lower themselves? I assume it's something to do with current passed from one rail to the other through the axle?

When I stuck train wheels on my DeLorean and rode it down the tracks it lowered the barriers automatically which caused a bit of a traffic incident in Oxnard.

There are sensor sections on both sides. If you short the tracks together with a large enough wire, it triggers the signal box. Actually learned this at the MIT swap fest when manning the back gate a decade ago. Got some cheap alligator clips and strung to them together, no luck.... Larger gauge copper did trigger it, and confused a ton of people when no train came by lol