The RS-11 was a road switcher introduced in 1956 as the successor of the RS-3. It used the new 251 engine instead of the 244. In this case, it had twelve cylinders and 1,800 hp. The customers had the choice between a high hood and a low hood variant. 334 were built directly by ALCO and 99 by Montreal Locomotive works. The largest customers were Norfolk & Western with 99 and Nacionales de México with 94. They were the last road switchers built by ALCO for the domestic market when production was completed in 1964. The MLW RS-18, of which 351 were built until 1968, was nearly identical to the RS-11. ALCO had also offered a six-axle variant as the RSD-12.