Learning the ["typical" US] Aspects

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Learning the Aspects

Abstract

Here are some catchy rules used when learning the common U.S. aspects. These rules work for the 'common' and 'sensical' versions of the signals below only, and even then, not in all cases. These rules used to work in Canada also until that country went over to the CROR system.

Rules to Use in Learning the Aspects

• Recall that "aspect" is how a signal looks, and "indication" is what it means. Each aspect also has a name. For example, an aspect might be: a yellow light over a green light over a red light. The aspect name might be "Approach Medium". The indication might be "Approach next signal at Medium Speed. Trains exceeding Limited Speed must immediately reduce to that speed."
• "If it's not all red, it's not red at all."

  This is an unfortunate development of this signalling scheme. The only U.S. railroad, to my knowledge, where red always means Stop is the Baltimore & Ohio (B&O). See the color-position-light signal page.

• Red is used as a placeholder. If any lamp is not red, no reds are significant. (This does not hold true for dwarf signals, where only nonflashing reds are insignificant.)
• The 3 arms correspond, essentially, to 3 speeds: top arm = full (allowed) speed, middle arm = Medium Speed, lowest arm = Slow Speed. Arms are "counted" and "added" from top to bottom; i.e., a 2-arm signal has positions for Full and Medium speeds. "Full" speed, for that location, is defined in the Employee Timetable and Special Instructions.
• If either green or yellow are present alone (with all other lamps being red placeholders), the position of the green or yellow tells you the speed the train is to be going NOW and through the interlocking. (The historical exception is Restricting). The color of the nonred lamp then provides block information: if green, there are 2 empty blocks (at least) and the signal name contains the word "Clear" after the speed. If the color is yellow, there is only 1 empty block and the signal name contains the word "Approach" after the speed.
• Thus, aspects whose names start with a speed are found at interlocking (controlled) signals. The speed indicates the speed the train must be going NOW, to make it through the switches of the interlocking.
• If yellow and green are present together, some sort of warning is being given about the next signal ("signal in advance"). The position of the yellow tells the allowed speed NOW (though this was later modified). The position of the green tells the allowed speed at the NEXT signal. Thus, if the yellow is on the top arm, one has 2 choices for the location of the green light: middle arm for Medium, lower arm for Slow. If the yellow appears on the middle arm, the green must be "bumped" to the lower arm, and flashing is then used to "upgrade" the Slow to Medium as necessary. Yellow is never allowed to be below green, except in one "exception" signal.
• Thus, aspects whose names start with "Approach" are found just before interlockings. They are used to indicate to the engineer the speed that applies at the NEXT signal, i.e., at the interlocking.
• When Limited Speed came along years later, it would have "made sense" to add a 4th arm, between the top and middle arms. Instead, to save on infrastructure, flashing of the middle arm was used to "upgrade" Medium to Limited.
• Number plates, "G"[rade], "P"[ermissive], "R"[estricting], and "A"[bsolute] plates do not change the meaning of any aspect except Stop-and-proceed. Thus, they are significant only for the Stop-and-proceed aspect. (Different roads were explicit to differing degrees in showing this in their books of rules.)
• Exceptions to these rules are much too plentiful. The exception to the rule that yellow must always be above a green has already been mentioned. There's even an exception to the rule that flashing lights upgrade the speed. As Dave Pierson mentioned, even the PC manual states that certain signal aspects are exceptions that "simply [must be] memorized".