MLW "S-12" Locomotives: Specs, Roster, History

Published: August 20, 2024

The S-12 - not to be confused with a similar model produced by Baldwin-Lima-Hamilton in the U.S. between 1950 and 1956 - was a switcher model manufactured by Montreal Locomotive Works (MLW) in 1958, tailored specifically for the Canadian National Railway.

The S-12 was built on the proven Alco (American Locomotive Company) design platforms, offering the robust performance typical of the builder's models from that period.

Sporting GE's rugged 731 traction motors it provided substantial tractive effort for switching duties. It was equipped with Alco's 539T turbocharged prime mover, a 6-cylinder, 4-stroke diesel engine known for its durability and ease of maintenance.

The railroad remained a loyal Montreal customer and purchased at least a few variants of nearly all of its cataloged models. CN was especially fond of Alco's turocharged engines. Interestingly, while Class 1's in the United States had largely phased out Alco-built models by the early 1980s they could still be found in service in Canada well into the 1990s.

With a combination of robust structural engineering and a reliable powertrain, the S-12 showcased MLW's commitment to manufacturing durable, high-performance locomotives aligned with the evolving needs of North American railroads.

The legacy of the S-12 is seen in its enduring service life, with CN's remaining in service for more than 20 years. It remains a noteworthy example of mid-20th century locomotive engineering, reflecting both the ingenuity and industrial prowess of its era.

Canadian National S-12 #8237 is seen here at Sarnia, Ontario on May 27, 1981 showing off the high riding underframe of these units. Doug Kroll photo.

Custom Designed Variants

Following the conclusion of MLW's production of the S-3 and S-4 in 1957 - the end of its standard switcher catalog - the company received four specialized orders from Canadian railroads, each for a unique switcher model. These included the S-7, S-10, S-11, and S-12. All were purchased by either Canadian National or Canadian Pacific.

The S-7 and S-12 were unique to CN, showcasing distinctive features in their design evolution. Early versions of S-2, S-3, and early S-4 were constructed with riveted carbodies. However, later S-4s, as well as the S-7, S-12, and S-13, featured more durable welded carbodies.

The S-12 was the third custom-built switcher order in the series—fourth in model sequence—and included a small roster of eleven 1,000 horsepower units for CN. These locomotives - road numbers 8235-8245 (construction numbers 82259-82268) - were constructed under specification DL-410 and completed between May 8 and August 18, 1958.

Spotting Features

A notable design change in these later models was the relocation of the number indicators from the side to the front of the long hood. Of particular note with the S-12s and S-13s was the inclusion of a single front radiator grille in place of the older dual side grilles.

The switchers also featured a higher underframe (often accompanied by a step near the front) and elevated cab roofs for enhanced insulation. The changes were subtle but these MLW builds were outwardly different than standatrd S series models.

The S-12 is noteworthy for pioneering the redesigned switcher carbody later used on the S-11 model. The only visible difference between the S-11 and the S-12 was the rectangular exhaust stack, accommodating the turbocharger's exhaust. The S-12 measured slightly longer, at 46 feet 3 ¾ inches (the S-11 measured 45 9 ¾ inches).

In terms of mechanical specifications, the S-12s shared much with the S-7s, including the 539 turbocharged engine, electric transmission, and 75:16 gear ratio. These custom-built locomotives underscored the blend of innovation and continuity in MLW's approach, ensuring enhanced operational efficiency for Canadian National.

Production Roster

Serial Number(s)	Completion Date(s)	Owner	Road Number(s)
82127	5/1958	Canadian National	8235
82259-82262	5/1958	Canadian National	8236-8239
82263-82265	6/1958	Canadian National	8240-8242
82266-82267	7/1958	Canadian National	8243-8244
82268	8/1958	Canadian National	8245

Data Sheet

Specifications: DL-410 (S-12)
Engine Builder	Dominion Engineering Works
Engine	4-cycle Model 539T 6L
Bore & Stroke	12" X 13 ½"
RPM (Maximum / Minimum)	740 / 315
Main Generator	GE - GT740
Horsepower	1000
Gear Ratio	75:16
Speed	60 mph
Trucks	4-Wheel
Configuration	B-B
Weight	246,000 lbs
Traction Motors	GE 731 (4)
Tractive Effort (Starting)	47,000 lbs at 25%
Tractive Effort (Continuous)	34,000 lbs at 5 mph
Auxiliary Generator	GE GMG490 GY424A
Air Brake	Westinghouse (26L)
Compressor	Westinghouse (3CD)
Exterior Dimensions
Total Length	46' 3 ¾"
Pilot to Pilot	37' 3"
Wheel Diameter	40"
Truck Wheel Base	8' 0"
Height to Top Engine Hood	12' 8"
Height to Top Cab Hood	15' 2"
Cab Width	10' 1"
Top Walkway Width	7' 9"
Walkway Width	5' 1"
Engine Hood Width	9' 11"
Center Bolster	22' 3"
Center Front Truck to Front Pilot	7' 6"
Center Rear Truck to Rear Pilot	7' 6"
Distance between Truck Centers	30' 3"
Minimum Turning Radius	50 degrees
Capacities
Fuel Oil	600 Gallons
Lubricating Oil	120 Gallons
Engine Cooling Water	92 Gallons
Sand Capacity	27 cubic feet

Later Years

Up until 1960, S-2s, S-3s, and S-4s operated throughout CN's yards across Canada. By 1961, however, these early models were predominantly allocated to CN’s Central and Eastern regions, where they were joined by the newer S-7s, S-12s, and S-13s.

From 1969 to 1975, CN retired all remaining S-2s, S-3s, and S-4s built before 1954, although some continued to serve on Grand Trunk Western (GTW) lines until 1983. By 1985, CNR had retired all remaining S-4s, S-7s, and S-12s, though 22 S-13s were upgraded and modified into the 8700, 300, and 100 series. The last S-13 was retired in 1997, marking the end of an era for these robust switchers.