Post Description

The Big Boys were designed to pull a 3,600 short ton (3,300 t) freight train over the long 1.14% grade of the Wasatch Mountains in Utah. Before their arrival, helpers were needed. Adding and removing helpers from a train slowed them down. For such locomotives to be worthwhile, they had to be faster and more powerful than slow mountain luggers like the earlier compound 2-8-8-0s that Union Pacific ("UP") tried after World War I. To avoid locomotive changes, the new class would need to pull long trains at sustained speed-60 miles per hour (100 km/h)-once past the mountain grades. Towards the end of the 4000's career (in the late 1950s) it was found that they could still pull more than their rated tonnage of 3,600 tons (3,300 t). Their ratings were increased several times until they regularly pulled 4,450 short tons (4,040 t) up the Wasatch grade, unassisted.

25 Big Boys were built, in two groups of ten and one of five. All were coal burning, with large grates to burn low quality Wyoming coal from mines owned by the railroad. 

Builder  American Locomotive Company
Build date  1941 (20), 1944 (5)
Total production  25
Configuration  4-8-8-4
UIC classification  (2'D)D2' h4
Gauge  4 ft 8 1?2 in (1,435 mm)
Leading wheel diameter  36 in (914 mm)
Driver diameter  68 in (1,727 mm)
Trailing wheel diameter  42 in (1,067 mm)
Wheelbase  72 ft 5.5 in (22.09 m)
Length  Locomotive: 85 ft 3.4 in (25.99 m)
Overall: 132 ft 9¼ in (40.47 m)
Width  11 ft (3.4 m)
Height  16 ft 2½ in (4.94 m)
Weight on drivers  540,000 lb (244,939.9 kilograms)
Locomotive weight  762,000 lb (345,637.4 kilograms)
Tender weight  342,200 lb (155,219.3 kilograms) (2/3 load)
Locomotive and tender combined weight  1,250,000 lb (566,990.5 kilograms)
Fuel type  Coal
Fuel capacity  30 short tons (27.2 t)
Water capacity  22,000 U.S. gal (83,000 l; 18,000 imp gal)
Boiler  95 in (2,400 mm)
Boiler pressure  300 lbf/in² (2.1 MPa)
Fire grate area  150 sq ft (14 m2)
Heating surface: Tubes and flues  5,035 sq ft (468 m2)
Heating surface: Firebox  720 sq ft (67 m2)
Heating surface: Total  5,735 sq ft (533 m2)
Superheater type  Type A
Superheater area  2,043 sq ft (190 m2)
Cylinders  Four
Cylinder size  23.75 × 32 in (603 × 813 mm)
Top speed  80 mph (130 km/h)
Tractive effort  135,375 lbf (602.18 kN)
Factor of adhesion  4.11