When in operation, the snowplow is pushed by several locomotives, depending on the snow conditions and the amount of drifting. As the plow train moves, often at speeds of up to 40 miles an hour, the engineer on the lead locomotive is unable to see the track ahead because of the snow being thrown away from the track by the plow. Before the advent of radios, communication was by whistle signals. The plow operator would use the following signals to tell the locomotive engineer what action to take.
One short whistle - stop at once
One short, one long, one short - move backward
Long, short, long - move forward
Thanks to the folks at Trackside Guide for additional information on the snowplow, and to Lewis E. "Gene" Green of El Paso, Texas for his contribution to the information on 55245.