Seaboard Air Line 4-8-2 "Mountain" Locomotives in the USA

Class M (Locobase 5008)

Much of the data comes from [] for 1929 locomotive diagrams. See also "Seaboard Air Line Mountain Type Locomotives", Railway Age Gazette, Volume 59, No 3 (16 July 1915), pp. 87-89; and "Mountain Type Locomotives for the Seaboard Air Line", Railway and Locomotive Engineering, Volume 28, No 7 (July 1915), p. 224. Works numbers were 54901-54910 in 1914 and 56630-56634 in 1917.

The first group of Mountains built for the Richmond-Hamlet-Atlanta service. According to the Railway Age Gazette, the particular section assigned to this new class was challenging. In the 160 miles between Richmond to Raleigh NC, "... there are several grades of 1.2 per cent, 2 1/2 miles long; and from Raleigh to Columbia. 207 miles, on which there are grades of 1.25 per cent, 3 1/2 miles long."

RAG's comment on the reason for resorting to a 4-8-2 sounds very similar to French reasoning behind adopting 2-8-2s: "The introduction of the Mountain type engines was because of the inability of the locomotives of the Pacific type to maintain a sufficient speed up the grades with 10 or more steel cars to avoid the necessity of exceeding the maximum speed limit of 50 miles per hour on other parts of the run."

And indeed,while the Pacific hauling an eleven-car train would slow to 18-20 mph (29-32 kph) before cresting some of the steeper grades, the M maintained 35-40 mph (56.5-64 kph) while hauling the same train. An added bonus was the 11% reduction in fuel consumption from the 13.5 lb per car mile (3.8 kg/car km) consumed by the Pacifics to the 12 lb/car mile (3.4 kg/car km) of the M class. Pacifics starting a train from a station on similar grades frequently had to take up the slack or even back off. RAG said of the M "the latter expedient has never been resorted to, and it is exceptional to have to take the slack in starting."

Firebox heating surface included 26 sq ft (2.42 sq m).of arch tubes.

These were the only Seaboard Mountains with Walschaerts gear. See Locobase 5009 for the 1923 Baker-gear batch.

Class M-1 (Locobase 5007)

Much of the data comes from [] for 1929 locomotive diagrams. Works numbers were 63188-63197 in February 1922.

Locobase 5008 for the first fifteen locomotives in this class delivered in 1914-1915.

Eight years after taking delivery of the M class, the Seaboard ordereed a second group of Mountains built for the Richmond-Hamlet-Atlanta service. They repeated the design of the first batch except for the substitution of Baker for Walschaert valve gear.

Firebox heating surface included 26 sq ft (2.42 sq m) of arch tubes.

Class M-2 (Locobase 225)

Data from tables in Locomotive Cyclopedia of 1930 and SAL 1 - 1948 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also DeGolyer, Vol 75, pp. 258+. (Thanks to Chris Hohl for spotting the 16-gallon tender capacity originally entered and querying it.) Works numbers were 58074-58083 in November 1924; 58681- 58682 in September 1925; 58724-58728, 58748-58751, 58760-58764 in October; 58992-58996 in January 1926; 59047-59051 in March.

These were enlarged M-class Mountains (see Locobase 5008 and 5007) with a larger boiler holding eleven more superheater flues and a commensurately larger superheating surface. The firebox heating surface area expanded as well, but the same system of arch tubes contributed a similar 26 sq ft (2.4 sq m) to the overall value. Also, the combustion chamber length increased by almost 30% and now contributed 109 sq ft (10.1 sq m) to direct heating surface area.. Taken with the 3" taller drivers and the 14" (356 mm) piston valves, the result is a design that emphasized steaming capacity over tractive effort.

With this batch, the SAL had all of the Mountains it would want. They remained in service until 1956.