+**131a** refers to a specific [signal aspect](/wiki/Signal_Aspect) used in North American railway operations. This instruction dictates that a train should approach the next signal at medium speed, prepared for a change in routing or a stop. It is a vital component of [railway safety](/wiki/Railway_Safety) and the precise management of train movements.
+## See also
+- [Railway Signal](/wiki/Railway_Signal)
+- [Train Control](/wiki/Train_Control)
+- [Rail Transport](/wiki/Rail_Transport)
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