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Locomotive Phases - MLW RS-10, RS-18

The MLW RS-10 was introduced in late 1954 and combined the basic mechanical components of the RS-3 (a 12-cylinder, 1,600-horsepower 244-series engine and 4-wheel Type B trucks) with a full-height carbody designed to more easily contain equipment such as dynamic brakes or a steam generator. Several of the carbody features were similar to the ALCO RSD-7, introduced in early 1954. In 1956, both ALCO and MLW introduced an updated model (the RS-11 and RS-18, respectively) in which the 244-series engine was replaced with a 12-cylinder 251-series engine producing 1,800 horsepower. While most RS-10's disappeared from service in the 1970's and 1980's, the improved reliability and parts availability of the 251-series engine led to a number of RS-18's continuing in service (often in rebuilt form) on regionals and shortlines well into the 21st Century.

The details I've listed below are based on a combination of information from CN and CP data books and measurements I made of an ONT RS-10 and a CN RS-18 preserved at Exporail. For simplicity, when describing "left" and "right" sides of the locomotive below, I'm assuming the perspective looking from the long to the short hood, as if the unit were operated short hood forward. In practice, most RS-10's and some RS-18's were built to operate long hood forward.

Phases - RS-10, RS-10S

The RS-10 was in production from late 1954 until early 1957. Although there were several variations in fuel tank size, dynamic brakes (in the short hood or the top of the long hood), presence or absence of a steam generator, and subsequent railroad modifications, there were few production variations that would warrant phases.

Partway through RS-10 production, the electrical system was upgraded from amplidyne to static generator excitation, improving reliability and prompting an updated "RS-10S" model name.

Transition: From RS-10 to RS-18

The RS-10 and RS-18 were substantially similar, sharing the same overall dimensions and the same general underframe construction. However, most of the discussions or references I've found describing the RS-10 and RS-18 go one step further, claiming that, other than a few minor differences in air filters, the carbodies between the two units were "identical". They were not.

Unlike EMD and later GE diesel engines, both the 244 and 251 series engines had the two cylinder banks slightly offset, with the connecting rods sitting side-by-side on the crankshaft. The central hood doors on ALCO and MLW RS-series units generally lined up with the engine power assemblies on each side of the prime mover.

On the 12-cylinder 244-series engine, the six power assemblies on each side were all evenly spaced about 16.5" apart. Externally, on the RS-10, the result was three pairs of doors on the left side 29" wide and 4" apart. On the right side, to account for the offset cylinder banks, the pair of engine room doors near the intercooler was widened by about 3", pushing the four other pairs of doors (two for the engine and two for the generator compartment) about 3" closer to the cab.

The 12-cylinder 251 engine had the power assemblies spaced farther apart (about 18") and had an additional couple of inches between the front and rear halves of the engine. Therefore, the three pairs of engine room doors on the RS-18 were considerably wider - 35.5", 34.5" and 32.25", again spaced 4" apart. The order was reversed on either side of the hood to account for the offset cylinder banks; in all cases, the largest of the three pairs of doors was to the left. The other hood doors - two pairs near the cab and two pairs under the intercooler - were narrower than on the RS-10 and were the same size and position on both sides of the hood.

The RS-18 also moved the cab and battery boxes about 4" toward the short hood. Visually, the front of the cab on the RS-18 was offset from the jacking pad above the truck, instead of near the middle of the jacking pad on the RS-10. Accordingly, the short hood was shortened and the long hood was lengthened between the cab and the first seam on the hood. The long hood seam near the cab, along with the radiator fan and shutters, were in the same position on the RS-10 and RS-18.

On units with dynamic brakes, the (early) RS-18 separated the two square air filters near the cab (which were grouped together on the RS-10) and the spacing of all the square filters was affected by the different hood door spacing. On account of the relocated cab on the RS-18, the handrail stanchion spacing was revised and the equipment box behind the cab was lengthened.

At the rear of the hood, the RS-10 had an external mechanism for opening and closing the radiator shutters, while the RS-18 moved the mechanism inside the hood with only a small lever and rod visible on the outside near the end of the hood. The intercooler was moved closer to the radiators, the rooftop hatch above the intercooler was shortened, and the exhaust stack housing was revised.

Phases - RS-18

The evolutionary changes made to the RS-18 were similar to those on the ALCO RS-11, described in the RS-11 Roster by David Thompson. I will be listing phases for the RS-18 here once I confirm all the detail changes.

Comparison with ALCO RS-11

The ALCO RS-11 was introduced around the same time as the MLW RS-18, and in fact the RS-18 was initially known (and continued to be listed by both CN and CP in diagram books) as the RS-11M. The RS-11 and RS-18 had the same hood door spacing as they used the same 12-251 engine. However, where the RS-18 continued to use the curved cab and plain hood corners of the RS-10, the ALCO RS-11 had sharper cab edges and notched hood corners for the sand hatches, numberboards and classification lights (as on the ALCO RSD-7).

The RS-11 also kept the rearward cab location of the RS-10 and had grabirons to access the higher sand hatches. The cab and battery boxes on the RS-11 were about 3.5" lower compared to the RS-10 and RS-18; this lower cab height (with the roof 9' 1" above the underframe and 3" above the hood) would be used on later ALCO RS units as well as the Century series from both builders.



In 2015 and 2017, I took ground-level measurements of CN RS-18 #3684 preserved at Exporail. In making the drawings, I made a few minor changes to the measurements (+/- 0.25") so all the totals add up.

Measurement notes:

Variations from the listed measurements:

MLW RS-18 1
MLW RS-18 2


I did not take measurements of ONT RS-10 #1400 at Exporail in 2015, and when I went back in 2017 it was no longer on public display. However, using a combination of diagrams, measurements of an MLW RS-2 (which shares much of the same door spacing), detail photos and prime mover dimensions - along with the many dimensions shared with the RS-18 - I was able to narrow down the hood dimensions of the RS-10 to a level of precision almost comparable to direct measurements.

The diagrams below list the dimensions that are different from the RS-18; the underframe is almost identical between the models. The RS-10 shown represents CN #3074-3093 (RS-10S) as-built and with Type B trucks, as opposed to the RS-18 drawings representing #3684 as modified and equipped with lightweight trucks. Otherwise, the drawings of the two models are as similar as possible (a late RS-10 and an early RS-18 built to the same CN specifications).

MLW RS-10 1
MLW RS-10 2


Canadian National, Chief of Motive Power & Car Equipment. (1964). Diesel Unit Data Book. Retrieved March 2010 from

Canadian National Railways Historical Association. (2017). RS-10/RS-10S. Retrieved January 2018 from

Canadian Pacific Railway, Chief of Motive Power & Rolling Stock. (1972). Classification and Dimensions of Diesel Units. Retrieved January 2018 from

Kerr, J. W. (1980). Illustrated CN and CP Rail Motive-power 1980-1981. Montreal, QC: Delta Publications Associates Division of DPA-LTA Enterprises Inc.

Thompson, D. Alco's RS11 - Original Owners. Retrieved January 2018 from

 Copyright © Michael Eby - Page code last updated 2018-01-12