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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?

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https://en.wikipedia.org/wiki/North_American_railroad_signals is a general introduction to the physical types of signals and the different signalling systems in use.

http://signals.jovet.net/rules/index.html has a "common reference guide" which describes the physical forms of different types of signals, plus guides to the signal codes used by different rail companies, including Union Pacific.

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  • $\begingroup$ Sometimes I see signs where a single pole contains two circular light fixtures, each of which with three lights on them (two on top and one on bottom). Other times I see two linear light fixtures, each of which with three lights on them. Is the fact that sometimes they're circular and sometimes they're linear purely an implementation detail? Or is that meaningful? I think I remember seeing a circular one where both top lights were red, which would confused me given your second link, but I could also just be remembering that incorrectly. $\endgroup$ – iAdjunct Jun 26 '16 at 4:47
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The signals mean exactly what you think they do: They tell the drivers of trains when it is safe to proceed, and often, how fast is safe. Everyone knows that trains are heavy and can take a long time to stop. Trains must be warned in advance long before they bump into one another so they can stop before that happens.

Railroad signals vary in many ways, even on the same railroad or the same route. There are different classes of signals which serve different purposes. The two main classes are absolute and permissive. Absolute signals require a train to stop and stay stopped when they are "red." Permissive signals allow a train to continue on at a slow, controlled speed. In the United States, these signals are more-or-less identical in form and appearance, except that permissive signals have a number plate indicating the track mile of the signal. Railroad signals, especially in the United States, can have many varied appearances, even of the same class. This is because signaling has evolved over the past 100+ years and different railroads and eras have held different views on how best to do it. Because of this, signals have formed a part of many railroads' identities because are unique to that railroad. Your description of the two different forms of signals near you sound just like you said, an implementation detail (of tradition).

I am the author of the second link about signaling posted by alephzero above. If you still have any more questions, feel free to ask.

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