The American Locomotive Company ("Alco") started building the RS-1 road switcher in 1940 and continued to build them into 1957. This was the longest production run of any single design in diesel engine history. The RS-1 had a 6-cylinder 539 motor in it, rated at 1,000hp, with a top speed of 60mph. It was produced with four wheel trucks and some six wheel trucks. The six wheel trucks were used on lighter rail where the weight had to be more distributed. The RS-1 was followed by the RS-2 (1,500hp) and the RS-3 (1,600hp), although they were produced simultaneously by Alco. The RS-1 could be used for yard switching, road service, or passenger trains (extensively used as such by the U.S. Army during WWII). They were outfitted by the manufacturer for one of those tasks.
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PRR RS-1 #5906 was the Pennsylvania Railroad's first diesel road switcher, which was bought in 1948. It was used to replace a steam engine in Baltimore, Maryland commuter service. During the Summer of 1950, the PRR bought 22 more RS-1 locomotives. Two more units were bought each year of 1951 and 1952. The PRR ran the engines with the long hood forward. All but one made it through to the Penn Central merger of 1968. In the PRR these were classified as "AS10am", except for the first one, which was an "AS10s". The first four characters stood for Alco, Switcher, 1000 horsepower.