Pennsylvania Railroad class D2

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.[4] They were the second standardized class of locomotives on the railroad and shared many parts with other standard classes.[1]

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)[1] 30 in (762 mm) (D2a)[2] Driver dia. 62 in (1,575 mm) (D2)[1] 68 in (1,727 mm) (D2a)[2] Wheelbase 22 ft 5 5⁄8 in (6.85 m)[2][3] Length 54 ft 5.44 in (16.60 m)[2][3] Width 9 ft 0.94 in (2.77 m)[2][3] Height 14 ft 8 in (4.47 m) (D2)[3] 14 ft 11 in (4.55 m) (D2a)[2] Adhesive weight 52,500 lb (23.8 tonnes) (D2)[3] 53,750 lb (24.4 tonnes) (D2a)[2] Loco weight 80,500 lb (36.5 tonnes) (D2)[3] 82,200 lb (37.3 tonnes) (D2a)[2] Tender weight 51,400 lb (23.3 tonnes) (D2 & D2a)[2][3] Total weight 131,900 lb (59.8 tonnes) (D2)[3] 133,600 lb (60.6 tonnes)[2] Tender type Eight-wheel with water scoop Fuel type Soft coal Fuel capacity 8,000 lb (3.6 tonnes)[3] Water cap 2,400 US gal (9,100 l; 2,000 imp gal)[3]

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 B2a in the post-1895 scheme; forty-five of them were constructed.[4][5]