Union Pacific Big Boy

Union Pacific Big Boy
Publicity photo of Union Pacific Big Boy #4000, the first of the class, taken in 1941
Type and origin Reference:[1] Power type Steam Designer Otto Jabelmann Builder American Locomotive Company (ALCO) Build date 1941 and 1944 Total produced 25
Specifications Configuration: ​  • Whyte 4-8-8-4  • UIC (2′D)D2′ h4g Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge Leading dia. 36 in (914 mm) Driver dia. 68 in (1,727 mm) Trailing dia. 42 in (1,067 mm) Minimum curve 288 ft (88 m) radius/ 20° Wheelbase Locomotive: 72 ft 5+1⁄2 in (22.09 m) Overall: 117 ft 7 in (35.84 m) Length Locomotive: 85 ft 3+2⁄5 in (25.99 m) Overall: 132 ft 9+1⁄4 in (40.47 m) Width 11 ft (3.35 m) Height 16 ft 2+1⁄2 in (4.94 m) Axle load 4884-1: 67,500 lb 4884-2: 68,150 lb Adhesive weight 4884-1: 540,000 lb 4884-2: 545,200 lb Loco weight 4884-1: 762,000 lb 4884-2: 772,250 lb Tender weight 4884-1: 427,500 lb 4884-2: 436,500 lb Total weight 4884-1: 1,189,500 lb 4884-2: 1,208,750 lb Fuel type Coal (No. 4014 converted to No. 5 fuel oil) Fuel capacity 28 short tons (25.4 t; 25.0 long tons) Water cap. 4884-1: 24,000 US gal 4884-2: 25,000 US gal Fuel consumption Up to 11 short tons of coal / hr Up to 12,000 US gal of water / hr Firebox: ​  • Grate area 150 sq ft (14 m2) Boiler 107 in (2,718 mm) (OD) Boiler pressure 300 lbf/in2 (2.1 MPa) Feedwater heater Elesco Type T.P. 502 Exhaust Steam Injector 14,000 US gal/hr capacity Heating surface: ​  • Firebox 704 sq ft (65 m2) (4884-1) 720 sq ft (67 m2) (4884-2)  • Tubes 967 sq ft (90 m2) (4884-1) 2,734 sq ft (254 m2) (4884-2)  • Flues 4,218 sq ft (392 m2) (4884-1) 2,301 sq ft (214 m2) (4884-2)  • Tubes and flues 5,185 sq ft (482 m2) (4884-1) 5,035 sq ft (468 m2) (4884-2)  • Total surface 5,889 sq ft (547 m2) (4884-1) 5,735 sq ft (533 m2) (4884-2) Superheater: ​  • Type Type E (4884-1) Type A (4884-2)  • Heating area 2,466 sq ft (229 m2) (Type E) 2,043 sq ft (190 m2) (Type A) Cylinders 4 Cylinder size 23.75 in × 32 in (603 mm × 813 mm) Valve gear Walschaerts Valve type Piston valves Valve travel 7 in (178 mm) Valve lap 1+3⁄8 in (35 mm) Valve lead 1⁄4 in (6 mm) Train heating Steam heat Loco brake Pneumatic, Schedule 8-ET Train brakes Pneumatic Safety systems Cab signals
Performance figures Maximum speed 80 mph (130 km/h) (design) Power output 6,290–7,000 hp (4,690–5,220 kW) @ 41 mph (66 km/h) (drawbar) Tractive effort 135,375 lbf (602.18 kN) Factor of adh. 3.99 (4884-1) 4.02 (4884-2)
Career Operators Union Pacific Railroad Class 4884-1, 4884-2 Numbers 4000-4019 (4884-1) 4020-4024 (4884-2) Last run June 21, 1959 (revenue service) Retired 1959–1962 Preserved Seven on static display and one (No. 4014) operational in excursion service Restored No. 4014; May 1, 2019 Disposition Eight preserved, remainder scrapped
Cost to build US$ 265,000 in 1941, equivalent to $5,489,457 in 2023

The Union Pacific Big Boy is a type of simple articulated 4-8-8-4 steam locomotive manufactured by the American Locomotive Company (ALCO) between 1941 and 1944 and operated by the Union Pacific Railroad in revenue service until 1962.

The 25 Big Boy locomotives were built to haul freight over the Wasatch Range between Ogden, Utah, and Green River, Wyoming. In the late 1940s, they were reassigned to Cheyenne, Wyoming, where they hauled freight over Sherman Hill to Laramie, Wyoming. They were the only locomotives to use a 4-8-8-4 wheel arrangement: four-wheel leading truck for stability entering curves, two sets of eight driving wheels and a four-wheel trailing truck to support the large firebox.

Today, eight Big Boys survive, with most on static display at museums across the United States. One of them, No. 4014, was re-acquired by Union Pacific, and between 2014 and 2019 was rebuilt to operating condition for the 150th anniversary of the first transcontinental railroad. It thus regained the title as the largest and most powerful operating steam locomotive in the world.