I'm asking this here because I really cannot find a better place to ask this.
Near my house (in the US), there is a pair of parallel train tracks operated by Union Pacific, primarily for cargo-carrying trains.
Along the track, there are various types of signals (for the trains): some are circular with three lights, some are linear with three lights (like a traffic light), and I think I've seen some with two lights. There's one pole on each side of the track, each with its own lights. Sometimes, on a pole, there are two sets of the lights - one above the other (I.e. Two traffic lights on the same vertical pole, and another identical one for the other set of tracks).
What do these signals mean? Why do they vary? How do I read them?