Wedge snowplows were the traditional mechanized method of clearing snow from
railway tracks. These pushed snow off the tracks, deflecting it to the side. Deeper drifts cannot easily be cleared by this method; there is simply too much snow to be moved. For this purpose, the rotary snowplow was devised. Rotary plows are often deployed when snow is too deep or hard-packed for traditional plows. The ability to operate slowly, as there is no requirement for train momentum to break up the snow, is often an advantage in mountainous regions, where a high speed derailment could be disastrous. Many rotary plows are not self-propelled, so one or more
locomotives are coupled behind it to push the plow along the line. An engine within the plow's carbody rotates the large circular assembly at the front of the plow. The blades on this wheel cut through the snow and force it through a channel just behind the disk to an output chute above the blade assembly. The chute can be adjusted to throw the snow to either the left or the right side of the tracks. An operator sits in a cab above and behind the blade assembly to control the speed of the blades and the direction of output from the chute. With the increasing prevalence of
Diesel locomotives,
multiple-unit train controls have been added to the cabs, so that the pushing locomotives can be controlled from the plow. In areas of particularly deep snowfall, such as California's
Donner Pass, railroads sometimes created a train consisting of a rotary snowplow at each end, with the blade ends pointing away from each other, and two or three locomotives coupled between them. With a plow on each end, the train was able to return to its starting location even if the snow covered the tracks it had just passed over. Such a train would also be able to clear multiple track mainlines efficiently, as it could make a pass in one direction on one track, then reverse direction and clear the next track. This practice became standard for the
Southern Pacific Railroad on Donner Pass following the January 1952 stranding of the
City of San Francisco train; during attempts to clear the
avalanches that had trapped the train, two rotary plows were trapped by subsequent avalanches, and the crew of a third was killed when their plow was hit by an avalanche. Rotary snowplows are expensive due to their high maintenance costs, which the railroad incurs regardless of whether they are needed in a given year. As a result, most railroads have eliminated their rotaries, preferring to use a variety of types of fixed-blade plows that have significantly lower maintenance costs, in conjunction with
bulldozers, which can be used year-round on maintenance-of-way projects. In addition, because rotaries leave an
open-cut in the snowbank that fixed-blade plows cannot push snow past, once rotaries have been used, they must be used for all further significant snowfalls until the
snowpack has melted. Since rotaries, which need energy to power the blades, also cost more to operate than fixed-blade plows, they are now generally considered to be a tool of last resort for the railroads that own them. and
Pingree in
North Dakota in February 1911. The few remaining rotary plows in North America are either owned by museum railroads or are kept in reserve for areas with poor road access and routine severe snowfall conditions. The largest remaining fleet of rotaries consists of
Union Pacific Railroad's six plows reserved for Donner Pass. Japan sees widespread use of rotary snowplows in its many mountain passes. == Power ==