I've watched many YouTube videos of Amtrak trains passing through signaled crossings. Why is it that at some crossings, the lights start flashing, the bells start ringing and the arms come down well before the train gets there, while at other crossings, those things occur just barely prior to the train's arrival? From what I've seen, it doesn't seem to be dependent on the speed of the train.
What's the explanation for that? Thanks!