(Train movement) – To report the departure from a station. O S was the telegraph code operators would send to the dispatcher to report that a train had departed a station. Used now to mean reporting a departure. OS is said to have many meaning, two of the more common interpretations are “On Sheet” (the time the dispatcher writes on the train sheet for the departure) and “Out of Station” (the time the train departs the station). On model railroads the crews may play the part of the operator and may report OS’s to the dispatcher.
Usage : Operator at Bess “Bess OS No 124 1242pm”
(Signal) – An “OS section” is the signal circuits at a control point (switch at the end of a siding or a crossover) that indicate occupancy by a train. They are called OS sections because when the train occupies them they are leaving, departing the station. When the leaving switch/control point is occupied the CTC machine records the OS or departure.