Union Pacific Challenger

Union Pacific Challenger
Union Pacific 3985 runs through Alton, Iowa, on October 1, 2008
Type and origin Power type Steam Designer CSA Class : Arthur H. Fetter 4664 Class : Otto Jabelmann Builder American Locomotive Company (ALCO) Build date 1936–1944 Total produced 105 + (6 Denver & Rio Grande Western)
Specifications Configuration: ​  • Whyte 4-6-6-4  • UIC (2′C)C2′ h4g Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge Driver dia. 69 in (1,753 mm) Wheelbase 60 ft 4+1⁄2 in (18.402 m) Engine 121 ft 10+7⁄8 in (37.157 m) Engine + tender Adhesive weight 4664-3/4 : 403,700 lb (183,115 kg) 4664-5 : 406,200 lb (184,249 kg) CSA-1/2 : 399,840 lb (181,364 kg) Loco weight 4664-3/4 : 627,000 lb (284,000 kg) 4664-5 : 634,500 lb (287,800 kg) CSA-1/2 : 566,950 lb (257,160 kg) Tender weight 4664-3/4 : 436,500 lb (198,000 kg) 4664-5 : 434,500 lb (197,086 kg) CSA-1/2 : 322,600 lb (146,329 kg) Total weight 4664-3/4 : 1,063,500 lb (482,400 kg) 4664-5 : 1,069,000 lb (484,890 kg) CSA-1/2 : 889,550 lb (403,493 kg) Fuel type Coal, oil Fuel capacity 32 short tons (29 t; 29 long tons) 6,450 US gal (24,400 L; 5,370 imp gal) UP3985 Water cap. 25,000 US gal (95,000 L; 21,000 imp gal) Firebox: ​  • Grate area 4664 Class : 132 sq ft (12 m2) (grate removed in 1990) CSA-1/2 = 108.25 sq ft (10 m2) Boiler 94 in (2,400 mm) Boiler pressure 4664-3/4/5 : 280 lbf/in2 (1.93 MPa) CSA-1/2 : 255 lbf/in2 (1.76 MPa) Heating surface: ​  • Firebox 4664-3/4 : 602 sq ft (55.9 m2) 4664-5 : 602 sq ft (55.9 m2) CSA-1/2 = 548 sq ft (50.9 m2)  • Tubes 527 sq ft (49.0 m2)  • Flues 3,687 sq ft (342.5 m2)  • Total surface 4,795 sq ft (445.5 m2) Superheater: ​  • Heating area 2,162 sq ft (200.9 m2) Cylinders Four Cylinder size 4664-3/4/5 : 21 in × 32 in (533 mm × 813 mm) CSA-1/2 : 22 in × 32 in (559 mm × 813 mm)
Performance figures Maximum speed 70 mph (110 km/h) Tractive effort 4664-3/4/5 : 97,352 lbf (433.04 kN) CSA-1/2 : 97,305 lbf (432.83 kN) Factor of adh. 4664-3/4 : 4.15 4664-5 : 4.17 CSA-1/2 : 4.11
Career Operators Union Pacific Railroad Class CSA-1, CSA-2, 4664-3, 4664-4, 4664-5 Preserved Two (Nos. 3985 and 3977) Restored No. 3985; April 1981 Disposition No. 3985 under restoration, No. 3977 on display, remainder scrapped

The Union Pacific Challengers are a type of simple articulated 4-6-6-4 steam locomotive built by American Locomotive Company (ALCO) from 1936 to 1944 and operated by the Union Pacific Railroad until the late 1950s.

A total of 105 Challengers were built in five classes. They were nearly 122 ft (37 m) long and weighed 537 short tons (487 tonnes). They operated over most of the Union Pacific system, primarily in freight service, but a few were assigned to the Portland Rose and other passenger trains. Their design and operating experience shaped the design of the Big Boy locomotive type, which in turn shaped the design of the last three orders of Challengers.

Two Union Pacific Challengers survive. The most notable is Union Pacific No. 3985, which was restored by the Union Pacific in 1981, then operated in excursion service as part of its heritage fleet program. Mechanical problems forced it out of service in October 2010; it was retired in January 2020 after the restoration of the 4-8-8-4 Big Boy 4014^[1][2] and eventually donated to the Railroading Heritage of Midwest America in 2022, where the locomotive is undergoing a second restoration.^[3] The only other surviving Challenger is UP No. 3977, which is on display in North Platte, Nebraska.