Locomotive classification on the Pennsylvania Railroad took several forms. Early on, steam locomotives were given single-letter classes. As the 26 letters were quickly assigned, that scheme was abandoned for a more complex system. This was used for all of the PRR's steam locomotives, and — with the exception of the final type bought (the E44) — all electric locomotives also used this scheme.
Contents
A: 0-4-0
Class A was the 0-4-0 type, an arrangement best suited to small switchers. Most railroads abandoned the 0-4-0 after the 1920s, but the PRR kept it alive for use on small industrial branches, especially those with street trackage and tight turns.
B: 0-6-0
Class B comprised the 0-6-0 type, the most popular arrangement for switchers on the PRR.
C: 0-8-0
Class C was assigned to the 0-8-0 type. These were very common on other railroads, but the PRR was not keen on them and only built a few. This was partly because the PRR used 2-8-0 "Consolidation" types for similar service.
D: 4-4-0
Class D was the 4-4-0 "American" type, the most common arrangement in 19th Century American railroading. 4-4-0s stayed in service on the PRR in secondary work later than on most other railroads, and three were in use until after World War II
E: 4-4-2
The 4-4-2 "Atlantic" type was assigned class E. The PRR was an enthusiastic user of the Atlantic type in flatter country, and built some of the most advanced Atlantics used in the United States.
F: 2-6-0
The 2-6-0 "Mogul" type was assigned class F. On the PRR, this type was used during the period of 1895–1925, approximately. They were mostly used to haul express freight, although some hauled suburban passenger trains.
G: 4-6-0
H: 2-8-0
I: 2-10-0
The 2-10-0 "Decapod" type was assigned class I. The PRR only owned one type of Decapod, class I1s, but they owned 598 of them, one of the largest classes of identical power in the United States.
J: 2-6-2 and 2-10-4
Class J was first used for two experimental 2-6-2 "Prairie" locomotives built by ALCO in 1905. These were both withdrawn from service by the mid-1920s. In 1942, the PRR built 125 2-10-4 "Texas" type locomotives based on C&O plans; class J now being unoccupied, it was reused for them. The PRR J1 was an improved version of its C&O counterpart with more pulling power.
K: 4-6-2
The PRR assigned class K to the 4-6-2 "Pacific" type. The Pacific was the most common type of passenger locomotive on the Pennsylvania.
L: 2-8-2
Class L was assigned to the 2-8-2 "Mikado" type.
M: 4-8-2
N: 2-10-2
O: 4-4-4
The 4-4-4 arrangement was rare anywhere, and on the PRR it was found only on eight experimental electric locomotives.
P: 4-6-4
The 4-6-4 arrangement was seen on the PRR only on electric locomotives. As a steam locomotive arrangement, it was poorly suited to the PRR's mountainous terrain, wasting much potential adhesive weight on non-driven wheels. That it was so widely used by the rival New York Central would also likely have factored against PRR adoption.
Q: 4-4-6-4 or 4-6-4-4
The Q class comprised what were effectively 4-10-4s with the driving axles split into two driven groups. The Q2 was the most powerful non-articulated steam locomotive ever built and also holds the record for highest horsepower recorded by any steam locomotive at 7,987 hp.
R: 4-8-4
The PRR never built any steam locomotives of the 4-8-4 "Northern" type, although the T1 duplexes were effectively a "Northern" with the driving wheels split into two groups.
S: 6-4-4-6 or 6-8-6
Both S class locomotives were originally intended to only have four leading and trailing wheels, but increases in weight required an additional axle at each end. In the case of the S2, it was due to wartime limits on the use of advanced steel alloys.
T: 4-4-4-4
The duplex-drive T1 was the final class of steam locomotive constructed for the Pennsylvania Railroad, and possibly the most controversial.
Compound classifications
The PRR classified articulated locomotives and joined locomotive units by using multiples of the previous classifications.
Articulated steam:
Articulated electric:
Non-standard electric classification
The horsepower-based designation unique to the E44 would be retained and expanded by Penn Central to cover the former New Haven EF4s (E33) and EP5s (E40).
Diesel classification
The Pennsylvania Railroad was slow to dieselize. By the end of WW2 they only had 18 units. However over the next 22 years they had acquired a total of 3005 units. They bought from all the manufacturers: Alco 516 units, Baldwin 643 units, EMD 1,479 units, Fairbanks-Morse 200 units, General Electric 145 units, and Lima 22 units. This diversity was a nightmare for the maintenance department as there was very little compatibility amongst the different builders.