Top header photo
 Trainiax logo

Locomotive Descriptions and Phases - ALCO/MLW RS-2, RS-3

sample photo

Roberval & Saguenay #20, an MLW RS-2 preserved at Exporail, seen in 2015.

The RS-2 was introduced by ALCO in 1946, one of several models to pioneer the company's new 244-series engine. Similar in concept to ALCO's first road switcher (the RS-1 from 1941) the RS-2 used a lowered hood with the cab offset from one end, and a straight underframe riding on four-wheel trucks. In contrast to ALCO's earlier models which employed riveted carbodies with sharp corners, the RS-2 used a mostly welded carbody with a distinctly rounded hood and cab. The model was initially known simply as the "1,500-hp road switcher" under specification E-1661, and the model designation of "RS-2" was retroactively applied a few years later.

Mechanially, the RS-2 was very similar to the FA-1 introduced at the same time. A 12-cylinder 244-series engine generated 1,500 horsepower and powered four GE 752-series traction motors in four-wheel drop-equalized ("Type B") road trucks. Late RS-2 production in 1950 (specification E-1661-C) saw the horsepower increased from 1,500 to 1,600. A steam generator (installed in the short hood) was optional for passenger service, but dynamic brakes were not available. The air reservoirs and battery boxes (and steam generator water tank, if so-equipped) were under the frame between the trucks, and the fuel tank was under the cab. A no. 6 distribution valve for the brake system was located on a box on the walkway (an extension of the fuel tank) next to the cab and short hood.

A handful of RS-2 units were produced in Canada by MLW, which were generally similar to their ALCO-built counterparts.

RS-2 Illustration


In 1950, shortly after the increase to 1,600 horsepower, several other electrical and mechanical upgrades led to the introduction of the RS-3 (specification E-1662). Externally, the most obvious change was the relocation of the fuel tank (from under the cab to under the frame) and battery boxes (from under the frame to walkway-mounted boxes next to the short hood). Dynamic brakes were optional and were visible externally by additional louvers and vents in the short hood. The presence of both dynamic brakes and a steam generator required the use of a full-height short hood; this relatively uncommon combination garnered the nickname "hammerhead".

Compared to the RS-2, the underframe was lengthened by 6" out from under the short hood, and the long hood was lengthened by 1 foot, pushing both hood ends 3" closer to the pilots. The extra length was added around the main generator (between the cab and engine compartment) and was likely to accomodate the auxiliary generator being mounted to the end of the main generator, rather than being mounted above it with a belt drive. The short hood was widened from 67" to 72", making it the same width as the long hood. Most of the rivets along the bottom of the hood edges were eliminated.

Several dozen RS-3's were built by MLW, mostly for Canadian railroads but also for an export order to Brazil. While earlier production was similar to concurrent ALCO production, later versions adopted a unique carbody air filter arrangement, with a row of square air filters arranged along the upper portion of the hood doors (similar to Phase 2 ALCO units).

RS-3 Illustration

RSC-2, RSC-3

A four-motor, six-axle version of both models was also available, riding on A-1-A trucks and known as model RSC-2 and RSC-3 respectively. Aside from the trucks and associated details (such as the traction motor air duct locations and handbrake chain) they were the same as their four-axle counterparts. As the units were a similar weight to the four-axle models, the extra unpowered axle on each truck helped to spread the weight on lightly constructed branch lines, but it came at the expense of a roughly 1/3 decrease in maximum tractive effort.

The truck design on the RSC-2 used a 10' 6" wheelbase with evenly spaced axles and single equalizers that extended inside the truck frame. Two sets of springs were located between the axles, sitting on top of the equalizers and underneath the truck frame. A single spring stack was located on the outer ends of the truck frame, with an extension of the equalizers pulling up on a rod inside the springs. Later versions used slightly heavier equalizers (curved rather than straight along the bottom edge) and different angled lifting tabs at the bottom of the journal pedastals.

On the RSC-3, a revised truck design was introduced that used double equalizers on the inner and outer faces of the truck frame. This new equalizer design provided several advantages:

Truck illustration

Relatively few RSC-2 and RSC-3 units were produced, with a number built for export markets and a handful of RSC-3's built by MLW in Canada for PGE. Domestic production went to MILW, SAL, SOO and UP. Many were converted to RS-2's or RS-3's by swapping in four-wheel trucks, in some cases (such as with PGE) as early as the mid 1950s. However, the truck design introduced on the RSC-3 would later see use on the MLW RSC-13 and RSC-24 built for CN, and ultimately (when those two models were retired) on re-trucked MLW RS-18's.

244 Engine

Prior to the Second World War, most ALCO locomotives were powered by the 539 series engine, a large, slow-speed inline design that, while proven in switching service, was not suited to further horsepower increases for road freight or passenger service. In response to the success of the EMD FT introduced in 1939, ALCO began work on a new higher-speed 4-stroke V engine in 1940, designated 241. Progress was slow, and when several weaknesses became apparent during testing in 1944, ALCO started work on a second engine design to similar specifications, known as the 244. The first 244 engine was ready for testing in 1945, and work on the 241 engine (initially intended to remain as a stopgap) was abandoned soon afterward.

In late 1945, before testing on the 244 had fully concluded, construction began on the first locomotive models intended to use the new engine. The first FA-1 entered service in early 1946, followed a few months later by the PA-1 and the RS-2. Problems soon arose with the cast crankshafts, which were replaced with forged versions later that year; those also initially suffered failures due to manufacturing defects. Issues with the exhaust manifold and air-cooled turbocharger (adopted from the 241) remained unresolved, involving both mechanical reliability and acceleration characteristics. In effect, much of the testing of the new engine design occured on locomotives already in service, with many components and entire engine blocks replaced under warranty.

After extensive development and incremental improvements, the air-cooled GE turbocharger was finally replaced by a GE-designed water-cooled turbo in June 1953, followed by an ALCO-designed version in September 1954. Externally, the change was evident in the exhaust stack being centered and crosswise, rather than offset and lengthwise. This last revision of the engine, designated 244H, was also marketed by ALCO as the 250 engine to dissociate it from the reliability problems of previous versions. It was also the standard to which earlier engines were modified or rebuilt; most earlier units were promptly upgraded with the new turbocharger design. While late-production 244 engines addressed many of the design's initial reliability issues, the engine was ultimately replaced (after thorough testing) by the 251 series engine in 1956, and the RS-11 replaced the RS-3 in ALCO's catalog.

In Service

The RS-2 predated EMD's introduction of a comparable road switcher (the GP7) by three years, and nearly 400 were produced over the four-year production run. The RS-3 further went on to become one of ALCO's best-selling locomotive models, with more than 1,300 produced by 1956. However, the unexceptional reliability and parts availability of the 244 series engine ultimately spurred many railroads to withdraw the units from service by the 1970s. Several railroads embarked on efforts to repower the locomotives with ALCO 251-series or EMD 567-series engines, with some (such as MKT) doing so as early as the late 1950s. Other roads (such as Delaware & Hudson) traded units back to ALCO in the 1960s where components were reused in the production of newer RS-series models. A few units nonetheless remained in service well into the 21st Century on smaller railroads.


RS-2, RSC-2

There were no major production changes that would warrant phases.


Widely circulated RS-3 phases appear to have originated in a series of detailed articles in Mainline Modeler from 1984 to 1986, and describe variations in the louvers and air filters on the carbody and battery boxes. While accurate, these phases are incomplete, as there were many other changes to the carbody and underframe over the course of production (some of which were structurally more significant than changes to louvers).

After debating how to incorporate these details into the existing phases, I've elected to follow the original phases as closely as possible, but with sub-phases added for the additional details. Since several changes appeared at once in late Phase 1b production, I've elected to split that off into Phase 1c. I've listed the Mainline Modeler phases for comparison.

As I have not found photos of every RS-3 order, some subphases may extend a month or two beyond the dates I've listed.

Phase 1a1 1a2 1b1 1b2 1c 2 3a 3b 3c 3d 3e
Dates 1950-06 1950-06 -
1951-05 -
1951-10 -
1952-07 -
1952-10 -
1953-11 -
1954-09 -
1955-03 -
1955-07 -
1955-09 -
Mainline Modeler Phase 1 1a 1b 2 3
Carbody intakes Tall louvers near
cab & radiators
Short louvers added
on central hood doors
Square / rectangular filters
at top of hood doors
Vertical groups of square filters
near cab & radiators
Battery box louvers Vertical Horizontal
Fuel tank vent pipe Extends approx. 44" above walkway Extends approx. 24" above walkway (late Phase 1b1)
Air pipe cooling manifold (1)No Yes
Phase 1a1 1a2 1b1 1b2 1c 2 3a 3b 3c 3d 3e
Short hood
handrail stanchions
2 on walkway, 1 on battery boxes 1 on walkway, 2 on battery boxes
Long hood
handrail stanchion spacing
49" - 5 x 50.5" - 47" 49" - 3 x 50.5" - 58" - 52" - 38"
(2nd and 3rd stanchions from cab moved toward cab)
Turbocharger Air cooled, offset lengthwise stack (as built) Water cooled, centered crosswise stack (after 1953-06)
(retro-fitted to earlier units)
Dynamic brakes
(if present)
Vertical louvers near end of short hood,
finned vent on top
Square intake near cab,
hinged vents with
rectangular brackets on top
Jacking pads Recessed rectangle Smooth,
slightly taller
Pilot to coupler
pulling face
23 7/8" 26 7/8"
Radiator intakes Smooth shutters, ridge around frame, shroud over shutter mechanism (2)
Phase 1a1 1a2 1b1 1b2 1c 2 3a 3b 3c 3d 3e


  1. Air pipe cooling manifold: Located on the right side underframe next to the air reservoir
  2. Radiator intakes, Phase 3c: Riveted shutters, no ridge around frame, exposed shutter mechanism (same as on RS-2)


The evolution of details in MLW production was generally similar to concurrent ALCO units. However, Phase 2 MLW units used a unique carbody filter arrangement similar to ALCO Phase 2 units, with six square filters along the upper portion of the hood doors. The longer underframe and reverted radiator shutters did not appear in MLW production.

Phase 1a 1b 1c 2a 2b
Dates 1951-05 -
1952 1953-07 -
1954-03 -
1954-08 -
Battery box louvers Vertical Horizontal
Fuel tank vent pipe Extends approx.
44" above walkway
Extends approx. 24" above walkway
Air pipe cooling manifold No Yes (except PGE Phase 1b-1c units)
Short hood
handrail stanchions
2 on walkway,
1 on battery boxes
1 on walkway, 2 on battery boxes
Long hood
handrail stanchion spacing
49" - 5 x 50.5" - 47" 49" - 3 x 50.5" - 58" - 52" - 38"
Turbocharger Air cooled, offset
lengthwise stack (as built)
Water cooled, centered crosswise stack
(retro-fitted to earlier units)
Carbody intakes Louvers 6 square filters
Dynamic brakes
(if present)
Vertical louvers near end of short hood,
finned vent on top
Square intake near cab,
hinged vents with
rectangular brackets on top
Jacking pads Recessed rectangle Smooth,
slightly taller
Phase 1a 1b 1c 2a 2b


Several options were available from the factory beyond what was offered on the base model:


Model Length between
coupler pulling faces
over pilots
Pilot to
truck center
Pilot to coupler
pulling face
RS-2 55' 5 3/4" 51' 6" 30' 0" 10' 9" 23 7/8"
RS-3 (Phase 1-3a) 55' 11 3/4" 52' 0" 10' 9" (front), 11' 3" (rear)
RS-3 (Phase 3b-3c) 56' 5 3/4" 26 7/8"


These dimensions are taken from ALCO mechanical drawings. Later units had the draft gear moved outward by 3" (increasing overall length by 6"). On many units with no steam generator, a lengthwise fuel tank used the same mounting points as the fuel and water tanks, for a tank length of 13' 10". The dimensions are as-printed; the drawing itself rounds them off to the nearest 1/4".



As the RS-2 and RS-3 were common in service and popular among modelers, they have been well-discussed in articles and forums over the years. In researching from sometimes informal sources, I've attempted to filter out errors or omissions that have been propagated over the years (such as dynamic brakes being stated as available on the RS-2, or the water-cooled turbo being described as an "option" rather than a production update). While I used a number of ALCO mechanical drawings when making drawings of these models, I've only listed the drawings here that informed the phase and dimension descriptions.

ALCO 244 Prime Mover [Online discussion]. (2021). Retrieved February 2022 from

Alco/GE air cooled turbochargers [Online discussion]. (2009). Retrieved December 2023 from

American Locomotive Company. (1946). 1500 H.P. Diesel Electric Road Switching Locomotive, E-1661-A. [Specifications].

American Locomotive Company. (1946). Cab Floor Frame 2213S52810 (RS-2). [Drawing].

American Locomotive Company. (1946). Erecting Apparatus Drawing 4311S95160 (RS-2). [Drawing].

American Locomotive Company. (1947). 1500 H.P. Diesel Electric Road Switching Locomotive, E-1661-B. [Specifications].

American Locomotive Company. (1950). Erecting Drawing - Elevation, 717 (RS-3). [Drawing].

American Locomotive Company. (1950). Operating Manual TP-401 for Model RS-3 Road Switcher.

Burns, A. (2023). Alco "RS2" Locomotives. Retrieved December 2023 from

Burns, A. (2023). Alco "RS3" Locomotives. Retrieved December 2023 from

Davis, W. (2010). ALCO-GE "1600". Retrieved December 2023 from

Hayden, B. (1980). ALCO 1600-H.P. Road Switchers. Model Railroader Cyclopedia - Volume 2, 122-123.

Kerr, J. W. (1983). Illustrated Treasury of MLW - ALCO to Bombardier Locomotives. Montreal, QC: DPA-LTA Enterprises Inc.

Nelsen, J. (1981, July/August). ALCO RSD4 and RSD5. Mainline Modeler, 44-55.

Nelsen, J. (1984, August). The ALCO RS3. Mainline Modeler, 42-53.

Nelsen, J. (1985, February). The Phase II and IIa ALCO RS3. Mainline Modeler, 54-59.

Nelsen, J. (1986, April). The ALCO RS3: Phase Three. Mainline Modeler, 60-65.

Nelsen, J. and Zenk, R. J. (1984, September). The ALCO RS3: Spotting Guide. Mainline Modeler, 40-45.

Shaughnessy, J. (1967). Delaware & Hudson. Berkeley, CA: Howell North Books.

Steinbrenner, R. T. (2011). The American Locomotive Company: A Centennial Remembrance. Warren, NJ: On Track Publishers, LLC.

VanBokkelen, J. Notes on Detailing B&M Diesel Locomotives. Retrieved December 2023 from