Class D14 on 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....

 was a type of 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...

 with a 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...

 wheel arrangement

Wheel arrangement

In rail transport, a wheel arrangement is a system of classifying the way in which wheels are distributed beneath a locomotive.. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and connections, with the adopted notations varying by country...

 in the Whyte notation

Whyte notation

The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early twentieth century encouraged by an editorial in American Engineer and Railroad Journal...

.
They were originally designated class P in the PRR's pre-1895 classification scheme. Twenty-two locomotives were built; six in 1893 with 78 inches (1,981 mm) driving wheel

Driving wheel

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

s, and sixteen in 1894 with 80 inches (2,032 mm) drivers, classified D14a.
Later, all sixteen class D14a were rebuilt to class D14b with 68 inches (1,727 mm) drivers for secondary service after they were replaced in top-flight express service, while three of the six class D14 were similarly rebuilt to class D14c.