The American Locomotive Company pioneered the multi-purpose "road switcher" design in 1941 with its 1000 hp RS-1. Alco's designers added a second, shorter hood to a basic switcher to make room for a steam boiler for passenger train heat. The short hood also afforded the crew additional accident protection. Smoother-riding trucks made Alco's new design suitable for the higher road speeds that would be daunting in a typical switcher.
In 1946, Alco cataloged the first six-axle version of its road switcher, the RSC-2. Designed for operation on the lighter rail found on branch lines and short lines, the engine used two more axles to spread out its weight. The trucks were a so-called "A1A" configuration: the two end axles had traction motors while the middle wheels were unpowered idlers.
In 1951, the Chicago and Northwestern asked Alco to build a six-axle version of its RS-3, this time with all axles powered. The body and the 1600 hp Alco 244 prime mover were the same as used in the RS-3, but the additional traction motors gave the new model RSD-4 stronger low-speed performance. The longer trucks were both smooth-riding and more suitable for lighter rail. The design proved to be a winner, and a dozen railroads purchased the RSD-4 and its near-identical twin, the RSD-5, which sported an improved GE generator.
Did You Know?
While they were the first to be cataloged, the RSC-2s were not the first six-axle Alco RS locomotives. Back in World War II, the first two years of RS-1 production had gone to the military. The 13 units that had been delivered to American railroads were requisitioned from their owners and, along with 144 additional engines, were sent to Iran, Russia, and U.S. Army posts. These RS-1s were fitted with three-axle trucks to make them suitable for lighter rail on overseas roads. A group that plied the Trans-Iranian Railroad became known as "the diesels that saved Russia" because they brought in food and other supplies after the Luftwaffe had crippled Russian shipping. These engines later became the prototype for Russia's own early diesels.