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 D5 (formerly Class G, pre-1895) comprised eighteen lightweight 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 for light duty, maintenance-of-way and branch-line service, constructed at the railroad's own Altoona Works during 1870–1873.

They shared many parts with other standard classes, although less so with the heavy 4-4-0s on account of their lighter build; instead, they shared some components with 0-6-0

0-6-0

Under the Whyte notation for the classification of steam locomotives, 0-6-0 represents the wheel arrangement of no leading wheels, six powered and coupled driving wheels on three axles, and no trailing wheels...

 switcher

Switcher

A switcher or shunter is a small railroad locomotive intended not for moving trains over long distances but rather for assembling trains ready for a road locomotive to take over, disassembling a train that has been...

 classes F and H (later B1 and B2).
The Class G locomotives had a straight-topped boiler, unlike the wagon-top of the other 4-4-0 classes.