The sensors don’t require alignment. It’s two wires (ok, six) to the rails. Put some voltage on one rail, and if those steel wheels are close enough, it’ll see some voltage on the other rail. Measure it to determine the train speed, reverse engineer when to do the whole signal sequence, and boom.

I say six wires because they have isolators on either side of the crossing itself, so there are three separate circuits: one for the crossing itself, one for this way, one for that way. The logic is built so if it senses a train coming from “this way”, then sees it on the crossing itself, then sees it on “that side”, then sees “this way” unoccupied, then sees the crossing unoccupied, it opens the crossing. Likewise, if it saw a train coming from “this way” and then senses that it has stopped (the voltage change has stopped), it opens the crossings.

If you’re curious: Danny Harmon of Distant Signal covers crossings