In 1956 ALCo introduced the 1800 hp, 12-cylinder 251B engine and mounted it in a new carbody to produce the RS-11. The carbody was very similar to that introduced by MLW on the RS-10 except that it had deep notches in the ends of the hoods where the number-boards were located. CN purchased 15 of these units for the DW&P in 1956 and numbered them DW&P 3600-3614 (MR-18a) They originally had lightweight trucks but these were later changed to AAR Type-B trucks. In January 1965, 3609-3614 were transferred to the CV and in 1968, they were returned to the DW&P. Beginning in 1979, all the RS11s were transferred to the Central Vermont and eight (3600-3605, 3611-3612) were painted in Central Vermont livery but retained their original numbers. 3609 was the first unit to be withdrawn and was dismantled in 1980 at St Albans VT.
An additional RS-11 (CV 3609:2) was purchased second hand from N&W in 1979. DW&P chop-nosed the short hood of 3608 at West Virginia in May 1979.
All RS-11s purchased new were built with 74:18 geared traction motors but many were later fitted with 65:18 geared traction motors that increased their top speed from 65 to 75 mph. They did not have a grille over the intercooler, instead they had chicken wire. There were three square carbody filters in a horizontal line and louvers in the short hood (I presume that they were fitted with dynamic brakes). By contrast, the former N&W unit, 3609:2, had five square carbody filters between the intercooler and the cab and there were no extra louvers in the short hood.
By 1988, all units were retired and later some units were sold to shortlines.