Pennsylvania Railroad locomotive classification

Class A was the 0-4-0 type, an arrangement best suited to small switcher locomotives (known as "shifters" in PRR parlance).

Most railroads abandoned the 0-4-0 after the 1920s, but the PRR kept it for use on small industrial branches, especially those with street trackage and tight turns.

Class B comprised the 0-6-0 type, the most popular arrangement for switcher locomotives on the PRR.

In 1942, the PRR built 123 2-10-4 "Texas" type locomotives based on C&O plans; class J now being unoccupied, it was reused for them.

As a steam locomotive arrangement, it was poorly suited to the PRR's mountainous terrain, wasting much potential adhesive weight on non-driven wheels.

The Q class comprised what were effectively 4-10-4s with the driving axles split into two driven groups.

Both S class locomotives were originally intended to only have four leading and trailing wheels, but for the S1 increases in weight required an additional axle at each end.

The duplex-drive T1 was the final class of steam locomotive constructed for the Pennsylvania Railroad, and possibly the most controversial.