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The simple pattern is like this for single-track, four-aspect signaling, train moving right to left (westward). FY = flashing yellow G-Train-R-Y-FY-G for the westward signals G-FY-Y-R-Train-G for the eastward signals If an APB-ABS system is used, all eastward signals will drop to red up to the next siding, which will have an absolute signal leaving each end -- APB (Absolute Permissive Block) is absolute for opposing trains, permissive for following trains. In Rule 251 double track, the westward signal pattern above will obtain for a westward train on the westward track. Rule 251 double track is not reverse signaled, so there are NO eastward signals present. The only trains are following trains, unless a train order (old days) or track warrant (today) authorizes a train to run wrong main. Some railroads wire the signals to present two reds behind a train, but this is fairly uncommon.
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