The Pennsylvania Railroad

Pennsylvania Railroad

The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....

's steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

 class D2 (formerly Class B, pre-1895) comprised twenty 4-4-0

4-4-0

Under the Whyte notation for the classification of steam locomotives, 4-4-0 represents the wheel arrangement of four leading wheels on two axles , four powered and coupled driving wheels on two axles, and no trailing wheels...

 locomotives intended for mountain passenger helper service, constructed at the railroad's own Altoona Works 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

Driving wheel

On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...

 for greater tractive effort

Tractive effort

As used in mechanical engineering, the term tractive force is the pulling or pushing force exerted by a vehicle on another vehicle or object. The term tractive effort is synonymous with tractive force, and is often used in railway engineering to describe the pulling or pushing capability of a...

 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

Firebox

In a steam engine, the firebox is the area where the fuel is burned, producing heat to boil the water in the boiler. Most are somewhat box-shaped, hence the name.-Railway locomotive 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 inches (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.