Southern Pacific 4-10-2 Locomotives in the USA

Class SP-1/SP-2/SP-3 (Locobase 288)

Data from 1930 Locomotive Cyclopedia tables and SP Menke All-Time Steam Loco Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also "The Powerful Three-Cylinder Locomotive of the Southern Pacific Co,", Railway and Locomotive Engineering, Volume XXXIX [39], No 1 (January 1926), pp. 1-5 and additional information in No 3 (March 1926), pp. ; and Albert W Bruce, The Steam Locomotive in America - Its development in the Twentieth Century (New York: W W Norton, 1952), p 301. Works numbers were (SP-1) 66107, 66206-66209 in April 1925; 66210-66216 in May; 66217-66220 in June. SP-2 numbers ran 66788-66805 in May 1926 and 66806-66810 in June. SP-3 numbers followed as 67412-67413 in July 1927 and 67414-67421 in August.

Menke's All-Time Steam Locomotive Diagrams compilation shows the following changes in heating surface areas in all three classes: Firebox heating surface measured at 401 sq ft (37.25 sq m, up 11 sq ft) and a commensurate increase in total evaporative heating surface. Superheating surface dropped by 116 sq ft (10.78 sq m) to 1,384 sq ft (128.52 sq m).

Each cylinder was served through an 11" (279 mm) piston valve. Valve motion had its maximum cutoff limited to 70%, a step permitted by an increase in boiler pressure over the earlier 2-10-2s. R&LE's 1926 report explains the benefits of such a limit beyond allowing the steam to expand on its own. "[W]ith the valve gear designed to give full port opening ...[at] 70%, the port opening is greater at any given cutoff ...This results in a wider port opening when the engine is hooked up, with a consequent increase in horsepower and hauling capacity at high speeds."

Note the inside cylinder's four-inch (102 mm) shorter stroke.The inside valve was operated by Gresley conjugated gear, the outside valves by Walschaert radial gear..As with all SP engines, this one was oil-fired. The wheel arrangement was given the name "Southern Pacific" on the Espee, "Overland" on the Union Pacific.

Special equipment included the Franklin C-2 trailing truck booster and Worthington 4-B feed water heater capable of processing 7,200 US gallons (27,252 litres) per hour.

HB Comstock contends that the SP 4-10-2s ran quite well. His assessment may well derive from Alfred W Bruce's 1952 encomium to the design that claimed "...the use of three cylinders permitted the power to be delivered directly to two driving axles with good balancing conditions in the 63-in driving wheels. The outside piston and main rods were unusually long but produced no ill effects."

R&LE claimed that for a given adhesion weight, three cylinders permitted the application of 15% more tractive effort without slipping than available to its two-cylinder counterpart. "The factor of adhesion of 3.75 for Southern Pacific type locomotives would be equivalent to about 4.25 in a two-cylinder type."

The result of this particular union of boiler and running gear: "Combustion conditions were excellent, and the engines performed well at operating speeds of from 30 to 35 miles per hour with the 63-in drivers." Bruce adds, however, "Long continuous operation at higher speeds ...increased the maintenance, as might be expected with the heavy middle-engine moving parts."

Drury (1993) notes that they were too rigid for the curves snaking through the Donner Pass, where they were first tried. After relocation to the Sunset Route east of Los Angeles, the 4-10-2s ran until the mid-1950s. It is there - between Roseville and Summit, Calif on a 2 1/2%, 80-mile grade -- Bruce says, "...the engines were remarkably successful because the operating condtions prevented excessive speeds and the even torque of the three cylinders prevented undue stalling at low speeds."

The original Vanderbilt cylindrical tender behind the 5000 held 12,000 US ga(45,420 litres) of water and 4,000 US gal (15,140 litres) of fuel oil. It weighed 113,000 lb (58,967 kg) empty and 244,900 lb(111,085 kg) loaded. Later FTTs were delivered with tenders that carried 150 more gallons of water and 912 additional gallons of fuel oil. They weighed 264,500 lb (119,975 kg).

The entire stud of Southern Pacfics remained in service until 1953-1954.