CNR purchased its first MLW road switchers in 1953, a batch of 18 1600 hp RS-3s powered by the turbocharged 12 cylinder 244 engine (Y-7-a, MR-16a after 1954). These locomotives had louvered engine compartment doors. They were numbered 7830-7847, renumbered 1800-1817 in 1954, and renumbered again in 1956 to 3000-3017. At least nine RS-3s including 3000-3008 were later fitted with six exposed carbody air-filters in a horizontal configuration (the modifications were made to units assigned to the Central Region).
The first six units 7830-7835 (later 1800-1805 and finally 3000-3005) did NOT have dynamic brakes whereas 7836-7847 (later 1806-1817 and finally 3006-3017) had dynamic brakes in the short hood. These later units had extra louvers in the access doors and an additional full-bodied, vertical row of louvers on each side of the short hood closest to the headlight. They also had the dynamic brake exhaust grids on the roof centred along the length of the short hood. The photos of 3000 and 3009 show the differences in the short hoods of MR16a units without and with dynamic brakes.
All RS-3s on the CNR system were equipped with water-cooled turbochargers and so had a centrally located, crosswise exhaust stack. Some units were fitted with a winterization hatch cover over the radiator roof fan and some received an extended exhaust stack to reduce the exhaust fumes that entered the cab. During their lifetime, the headlights on many units were changed to twin sealed-beam headlights from the single headlight with two bulbs and the bell was mounted on the long hood.
In 1954 CNR purchased 23 more RS-3s and all units in this batch had a horizontal row of six exposed filters on either side of the hood. They were originally numbered 1818-1840 (MR-16b) and renumbered 3018-3040 in 1956. According to the CN Diesel Unit Data Book all units except 1818 (3018) and 1819 (3019) had dynamic brakes. The arrangement of the louvers and dynamic brake exhaust grids were different from those on the earlier dynamic-brake equipped units. There was no full-bodied, vertical row of louvers ahead of the access doors. Instead the forward access door had extra louvers so that there was a full bodied set of louvers. Also, the dynamic brake exhaust grids on the roof were closer to the headlight along the length of the short hood necessitating a raised section of the roof edge that broke the seamless rounded contour. The photos of CNR3019 and CN3022 show the differences in the short hoods of MR16b units without and with dynamic brakes.
In 1954, CNR purchased, from ALCo, two RS-3s fitted with steam generators for the Central Vermont and numbered them CV1859-1860 (MR-16c). On each side of these units were five vertically arranged carbody filters in two of the engine compartment doors. Later, 1859-1860 were renumbered to CV 3900-3901 and then became CN 3900-3901.
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