Pennsylvania Railroad class D2

PRR D2 & D2a
Class B A #138 (later class D2a)
Type and origin Power type Steam Builder PRR Altoona Works Build date 1868–1872 (D2); 1881–1882 (D2a) Total produced 20 (D2); 45 (D2a)
Specifications Configuration: ​  • Whyte 4-4-0  • UIC 2′B Gauge 4 ft 8+1⁄2 in (1,435 mm) Leading dia. 28 in (711 mm) (D2) 30 in (762 mm) (D2a) Driver dia. 62 in (1,575 mm) (D2) 68 in (1,727 mm) (D2a) Wheelbase 22 ft 5+5⁄8 in (6.85 m) Length 54 ft 5.44 in (16.60 m) Width 9 ft 0.94 in (2.77 m) Height 14 ft 8 in (4.47 m) (D2) 14 ft 11 in (4.55 m) (D2a) Adhesive weight 52,500 lb (23.8 tonnes) (D2) 53,750 lb (24.4 tonnes) (D2a) Loco weight 80,500 lb (36.5 tonnes) (D2) 82,200 lb (37.3 tonnes) (D2a) Tender weight 51,400 lb (23.3 tonnes) (D2 & D2a) Total weight 131,900 lb (59.8 tonnes) (D2) 133,600 lb (60.6 tonnes) Tender type Eight-wheel with water scoop Fuel type Soft coal Fuel capacity 8,000 lb (3.6 tonnes) Water cap. 2,400 US gal (9,100 L; 2,000 imp gal)

The Pennsylvania Railroad's steam locomotive class D2 (formerly Class B, pre-1895) comprised twenty 4-4-0 locomotives intended for mountain passenger helper service, constructed at the railroad's own Altoona Works (now owned by Norfolk Southern) during 1869–1880. They were the second standardized class of locomotives on the railroad and shared many parts with other standard classes.

This design differed from the Class A (later D1) mainly in its smaller drivers for greater tractive effort in mountainous terrain. Like all the early standardized 4-4-0s on the PRR, the Class B had a wagon-top boiler with steam dome and a firebox between the two driving axles.

In 1881, the PRR took the Class B design and modified it to produce more locomotives for express passenger service, with 68-inch (1,727 mm) drivers like the earlier Class A. These new locomotives were designated Class B A, and were classified as D2a in the post-1895 scheme; forty-five of them were constructed.