Yellow Aspect: Caution, proceed and preparing to stop at next signal.
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The most constant signals and the easiest to understand are single-head block signals. Red means stop; green means proceed, and yellow means caution or approach, usually indicating that the next signal is red.
The signal lights usually have four indicators: Green: clear to go. Double yellow: still clear to go, but indicates that the following signal may be a warning. Yellow: warns that the next signal may be red. Red: means stop or danger ahead.
The orange flag indicates that a train should be prepared to stop due to conditions it will be approaching. The yellow and red flag indicates that there is a temporary speed reduction followed by a stop signal. The yellow and green flag indicates that there is a temporary speed reduction, followed by an all clear.
Blue Signal DisplayBlue signals displayed in accordance with §§218.25, 27, or 29, signify that workers are on, under, or between railroad rolling equipment. When so displayed: The equipment may not be coupled to. The equipment may not be moved (see exceptions in §218.29).
Reading Color Light SignalsGreen indicates clear, the train can proceed. Yellow indicates approach, but at a restricted speed. Be prepared to stop at the next signal. Red indicates stop, the block is currently occupied.
Absolute signals are sometimes also known as “interlocking plant signals,” or more commonly today, “control points.” They received the name because these signals are typically operated in a control center by members of the Centralized Train Control (CTC).