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I'm surprised no one has mentioned the differences between absolute and permissive signals. From the standpoint of looking at signals to know if a train is coming, learning the difference between those two helped me tremendously. As many folks have already noted, there are many, many nuances from one signalling system to another. Basically: Absolute signals are generally found on lines with Centralized Traffic Control (CTC). A signal is absolute when it is sufficient to govern train movement by itself. An absolute signal will always display its most restrictive aspect ("Stop", all red lights), unless the dispatcher has lined up a train. So, if you're on a railroad that uses CTC and you see nothing but red signals, it's a safe bet there aren't any trains around. On the other hand, if you see anything other than red, then there's a train approaching that signal . . . somewhere. Permissive signals are found on railroads using Track Warrant or Direct Traffic Control. Permissive signals DO NOT have the authority to govern train movements. Trains can only move when given proper authority by the dispatcher (receiving DTC Blocks or a Track Warrant). Generally speaking, the most restrictive aspect that a permissive signal can display is "stop and proceed at restricted speed." Unlike their absolute counterparts, permissive signals often default to the "clear" aspect. In many cases, permissive signals are used simply as a means to show track occupancy. Thus, if you're on a railroad that uses DTC or TWC, and you see a red signal, it likely means that the next section of track (block) is occupied by a train. On the other hand, if all the signals are clear, then the nearest train is at least three blocks away. I hope that all makes sense and that it's helpful. If it's a little too basic of information, I apologize. I'm new here. Scott Lothes Cleveland, Ohio
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