ALCO S-2 and S-4

ALCO S-2 and S-4
Grafton and Upton Railroad #1001, an S-4, rests in Hopedale, MA on 11 May 2008
Type and origin Power type Diesel-electric Builder ALCO and MLW Model S-2 and S-4 Build date S-2: August 1940 (1940-08) – June 1950 (1950-06) S-4: June 1949 (1949-06) – August 1957 (1957-08) Total produced S-2: 1502, S-4: 797
Specifications Configuration: ​  • AAR B-B Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge Trucks S-2: Blunt S-4: AAR type A Wheel diameter 40 in (1,016 mm) Minimum curve 50° or 118.31 ft (36.06 m) Wheelbase 30 ft 6 in (9.30 m) Length 46 ft (14.02 m) Width 10 ft 2+1⁄2 in (3.11 m) Height 14 ft 6 in (4.42 m) Loco weight 230,000 lb (100,000 kg) Fuel capacity 635 US gal (2,400 L; 529 imp gal) Prime mover Alco 539T Engine type Inline 6 Four-stroke diesel Aspiration Turbocharged Displacement 1,595 cu in (26.14 L) per cylinder 9,572 cu in (156.86 L) total Generator GE GT 553-A Traction motors (4) GE 731 Cylinders 6 Cylinder size 12+1⁄2 in × 13 in (318 mm × 330 mm)
Performance figures Power output 1,000 hp (746 kW) @ 740 rpm Tractive effort 57,500 lb (26,100 kg)
Career Locale United States, Canada, Mexico, Australia

The ALCO S-2 and S-4 are 1,000-horsepower (746 kW) diesel electric switcher locomotives produced by ALCO and Canadian licensee Montreal Locomotive Works (MLW).^[1]

Powered by turbocharged, 6-cylinder ALCO 539 diesel engines, the two locomotives differed mainly in their trucks: the S-2 had ALCO "Blunt" trucks; the S-4, AAR type A switcher trucks. A total of 1,502 S-2s were built from August 1940 to June 1950; 797 S-4s were built from June 1949 to August 1957. The S-4 was first produced in Canada, with ALCO production beginning in June 1949.

The S-2 and S-4 were designed as rail yard switchers, meant to replace older, less efficient, and more demanding steam switchers.^{[citation needed]} They were a success, with many remaining in service today.^[2]

The locomotives' exterior was styled by ALCO engineer Ray Patten, who used curves in a mild application of Art Deco principles.

The S-2 and S-4 are distinguishable externally from the very similar S-1 and S-3 660 hp (492 kW) switchers in that they have a larger exhaust stack with an oblong base and a larger radiator shutter area on the nose sides. The S-1/S-3 radiator shutter area is taller than it is wide, while the S-2/S-4 radiator area is wider. The larger stack is due to turbocharging. The carbody and cab of late S-2s are nearly indistinguishable from those of S-4s. Hence, a truck swap can cause many to misidentify a unit.