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Locomotive Descriptions and Phases - ALCO RSD-12

The ALCO RSD-12 was introduced in 1956 as a replacement for the RSD-5 and as a six-motor version of the concurrent RS-11. It used a 12-cylinder 251-series engine developing 1,800 horsepower.

Visually, the RSD-12 featured a clean-lined carbody, with full-height hood and large vertical radiator intakes at the end with a single large radiator fan. The overall length was increased by 14" compared to the RS-11, with the hood lengthened immediately behind the cab. Early units had a rectangular chickenwire grill over the intercooler intakes covering an air cooling pipe manifold from the air compressor, located about 3/4 of the way along the long hood; this was replaced with a group of three square air filters on later units. The dynamic brakes were located in a raised housing along the center of the roof; units so-equipped had additional air intakes behind the cab (early units) or above the engine compartment doors (later units). A high short hood was initially standard, but many later-production units were built with a low short hood. The hood corners had 45-degree notches for the number boards and classification lights.

Under the frame, the trucks were ALCO's drop-equalized Tri-mount design with unequal axle spacing (5' 7" outboard, 6' 11" inboard) to accommodate the orientation of the traction motors. The "Tri-mount" designation came from the use of a central pivot located just outboard of the middle axle, combined with two sliding bearings on the truck frame between the inner axles; this 3-point support system reduced weight transfer under high tractive effort situations. Most RSD-12's used the later version of this truck with two brake shoes per wheel and two brake cylinders at the upper corners, but SP units used trade-in trucks that had single-shoe brakes and a single central brake cylinder. The fuel tank and air reservoirs were centered between the trucks, with the fuel tank extending out under the air reservoirs and bevelled along the bottom edge.

A total of 171 RSD-12's were built up to 1963. More than half went to Mexico, to Ferrocarril del Pacifico and Ferrocarriles Nacionales de Mexico, along with 10 built by MLW for RFFSA in Brazil. American production went to CO, LSI, NKP, PRR and SP; there was no MLW version sold in Canada. Most remained in service up to the late 1970's or early 1980's; four have been preserved (NKP 329 and three former SP units).

Related models


As the RSD-12 was a six-axle version of the RS-11, the two models were very similar and shared most of the same general construction and "phase" details. The underframe on the RSD-12 was lengthened by 14" compared to the RS-11; it used the same walkway height but had slightly higher clearance underneath for the Tri-mount trucks. The underframe construction was substantially heavier, with two 1.75" plates along the bottom and a 1" or 2" plate along the top (compared to a 1" and 0.5" plate on the bottom and a thin plate on the top on the RS-11). The walkway side frame (excluding the top plate) was 1" narrower than on the RS-11, and the air reservoirs were mounted about 2" lower, exposing the top edge of the tanks. The steps and draft gear were largely the same as on the RS-11. The jacking pads above the trucks were in line with the truck pivot pin (22" outboard of the center axle) and included a curved lifting bracket wrapped underneath the top edge of the truck frame.

Above the walkway, all of the 14" length increase over the RS-11 was added immediately behind the cab - about 9.5" between the cab and the first hood seam, and about 4.5" between the seam and the first hood door. All the rest of the short hood, cab and long hood (including the hood-to-pilot distances) was the same as on the RS-11, with the following exceptions:


The RSD-5 preceded the RSD-12 and used ALCO's earlier carbody design with rounded corners and a lower hood height (most well-known on the RS-2 and RS-3). The RSD-5 had 8" shorter truck centers and was 1' 8" shorter over the pilot plates; the additional length on the RSD-12 was in part due to the clearance required for the outer brake shoes on the trucks.

RSD-7, RSD-15

The RSD-15 was produced at the same time as the RS-11 and RSD-12, but was powered by a 16-cylinder (rather than 12-cylinder) version of the 251 engine rated at 2,400 horsepower. While it shared many general design characteristics with the RSD-12, it was more than 8 feet longer and had notably longer hoods. It was derived from the very similar RSD-7, an earlier model powered by the 244-series engine, which pioneered the general frame and carbody design used in subsequent 251-powered RS and RSD models.


Model Overall Length Length Over Pilots Truck Centers
RSD-5 55' 11 3/4" 52' 0" 34' 9"
RS-11 56' 11 3/4" 52' 6" 31' 0"
RSD-12 58' 1 3/4" 53' 8" 35' 5"
RSD-7, RSD-15 66' 7" 62' 3" 43' 6"


These phases are of my own making. I compared photos of contemporary RSD-12 and RS-11 units against the phases I had already compiled for the RS-11 - which were in turn based on the RS-11 Roster compiled by David Thompson.

See also RS-11 phases.

Phase 1a1 1a2 1a3 1b 2a 2b 3a 3b
Dates 1956-04 -
1957-02 -
1957-12 -
1958-08 1958-09 -
1959-11 -
1961-02 -
1961-12 -
45-degree handrail braces No Yes No
Air reservoir supports Attached to U-shaped
connector below side sill
Attached to straight connector with a notch in the side sill
Radiator intakes Narrow frame, 24 shutter vanes with no external clips,
control box at top right corner
Wider, bolted frame, 18 shutter vanes with external rod clips,
control mechanism at top corner facing the cab
Coupler pocket Cast Fabricated (except LSI 1803)
Air intakes behind cab 3 square intakes (units with dynamic brakes only) 1 + 2 square intakes with outer frame Rectangular group of 3 intakes
Middle hood air intakes 3 square intakes above center doors 3 square intakes with outer frame
(units with dynamic brakes only)
2 narrower rectangular intakes
(units with dynamic brakes only)
Phase 1a1 1a2 1a3 1b 2a 2b 3a 3b
Equipment box behind cab Yes (engineer's side only) No (except FCP units)
Handrail stanchions Square, fastened to walkway with L brackets
43" - 54" - 54" - 51" - 51" - 52" - 52" spacing
Cylindrical, fastened directly to walkway
44" - 44" - 44" - 60" - 56" - 52" - 52" spacing
Hood doors Equal height Taller engine compartment doors (middle 6)
(as on RS-32 and RS-36)
Rear hood intercooler intake Rectangular chickenwire grill covering air cooling pipe manifold Narrower rectangular group of 3 intakes
Hand brake wheel Recessed Surface-mounted
(also appeared on FCP 515-517 in 1961)
Phase 1a1 1a2 1a3 1b 2a 2b 3a 3b



ALCO Products Inc. (1959). Operating Manual 1800 hp road switchers models RS-11 and RSD-12.

Peck, D. (1987, September). ALCO's RSD-12. Mainline Modeler, 58.

Norfolk and Western Historical Society. (2019). Archives Database. [RSD-12 database search]. Retrieved November 2023 from

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