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Locomotive Descriptions and Phases - MLW RS-10, RS-18

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Preserved CN RS-18 #3684, seen with the hood doors open and the engine running for a demonstration on one of Exporail's "Diesel Days" in 2015.

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 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.

Typical spotting features were a clean-lined hood with a single radiator fan and tall radiator shutters near the end, 2-axle trucks (either Type B or a similar but slightly shorter lightweight version), several square air filters along the top of the carbody, and a raised intake housing ahead of the radiators (covering an intercooler pipe manifold from the air compressor, previously exposed under the walkway on the RS-3).

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 GE FDL diesel engines, both the 244 and 251 series ALCO 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, which meant the doors were different on each side of the hood, as well as different between locomotive models with either engine type.

On the 12-cylinder 244-series engine, the six power assemblies on each side were all evenly spaced 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 air intercooler intake was widened by 3", pushing the four other pairs of doors (two for the engine and two for the generator compartment) 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. As a result, the three pairs of engine room doors on the RS-18 were more than a foot wider overall - 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. (The only exception to the forward cab location I've found has been PGE's first pair of RS18's, built in 1957 with the cab position of the RS-10.)

On units with hood-mounted 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 wider engine compartment doors. Units with dynamic brakes in the short hood had an arrangement of doors and louvers on the sides of the short hood similar (but not identical) to the RS-10. 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 air 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

I have based the main phase divisions of the RS-18 (between 1, 2 and 3) on the RS-11 Roster compiled by David Thompson. While the evolutionary changes made to the RS-18 were similar to those of the RS-11, several of the details within sub-phases were different.

Phase 1a 1b 1c 1d 2a 2b 2c 3
Dates 1956-12 -
1957-12 -
1958-03 -
1958-06 -
1959-05 -
1959-06 -
1960-01 -
1964-08 -
Coupler pocket Cast Fabricated
Equipment box behind cab Both sides Right side only None
Jacking pads Tall, rounded corners Shorter, square corners
Front hood air intakes 2 separate filters 3 grouped filters
Handrail stanchions Square, bolted to L-brackets on walkway L-channel, bolted to walkway side frame
Pilot footboards Fabricated sections bolted to vertical pilot plate Angled sheet wrapping under footboard step
Intercooler intake Raised louvered housing Flush group of 3 filters
Hood, dynamic brakes Short doors, flat dynamic brake housing, square intakes above center doors
RS-32 styleTaller hood doors, raised grill on dynamic brake housing, shorter, rectangular intakes above center hood doors
Phase 1a 1b 1c 1d 2a 2b 2c 3

Other details

In 1975, CN rebuilt several dozen RS-18's with A1A trucks from retired RSC-13's and RSC-24's and numbered them #1750-1787. The engine was derated to 1,400 horsepower and they were commonly known as an "RSC-14", although CN diagrams list them as an RSC-18m. All were rebuilt from the last two orders of CN RS-18's (#3843 and up) and were used on light-rail branch lines in the Maritimes. Other than the different trucks and a new brake chain housing, they retained their previous overall appearance and phase spotting features.

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. On the RS-10 and RS-18, the cab roof was the same height above the frame as on the RS-3 but the battery boxes and sub-base were about 3.5" higher; the RS-11 combined the cab dimensions of the RS-10 and RS-18 with the lower battery box/sub-base height of the RS-3, lowering the cab roof relative to the frame by 3.5". 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 details photos of #1400, diagrams, measurements of an MLW RS-2 (which shares much of the same door spacing) 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 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 #3677-3700 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


In summer 2018 I measured CN RSC-14 #1754, preserved at the New Brunswick Railway Museum. Conveniently, not only does this unit differ from #3684 at Exporail by the swap to A1A trucks, it also (like all RSC-14's) is a later-production unit with transverse fuel tank/air reservoirs and L-channel handrail stanchions, thereby filling in many gaps in my RS-18 measurements.

Measurement notes:



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