PRR locomotive classification

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.

PRR steam and electric classification

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.

* A1
* A2
* A3
* A4
* A5s

B: 0-6-0

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

* B1
* B2
* B3
* B4
* B5
* B6
* B7
* B8
* B21
* B22
* B23
* B28s
* B29

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.

* C1
* C29
* C30
* C31

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

* D1
* D2
* D3
* D4
* D5
* D6
* D7
* D8
* D9
* D10
* D11
* D12
* D13
* D14
* D15
* D16
* D21 - ex-Vandalia Railroad, built by Baldwin 1871-1873
* D22 - ex-Vandalia Railroad, built by Pittsburgh 1883-1889
* D23 - ex-Vandalia Railroad, built by Pittsburgh 1884
* D24 - ex-Vandalia Railroad, built by Rogers 1887
* D25 - ex-Vandalia Railroad, built by Pittsburgh 1890
* D26 - ex-Vandalia Railroad, built by Schenectady 1895-1899
* D30 - ex-Toledo, Walhonding Valley and Ohio Railroad, built by Rome 1890
* D31 - ex-Toledo, Walhonding Valley and Ohio Railroad, built 1892-1895
* D32 - ex-Toledo, Walhonding Valley and Ohio Railroad, built by Baldwin 1892
* D33
* D34 - ex-Cleveland & Marietta Railroad, built by Rome 1885
* D35 - ex-Cleveland & Marietta Railroad, built by Rome 1885
* D36 - ex-Cleveland & Marietta Railroad, built by Rogers 1885-1886
* D37 - ex-Cincinnati, Lebanon, & Northern Railroad, built by Baldwin 1894
* D38 - ex-Cincinnati, Lebanon, & Northern Railroad, built by Pittsburgh 1899
* D39 - ex-Grand Rapids & Indiana, built by Baldwin 1892-1893
* D61 - ex-Cleveland Akron & Columbus Railway, built by Baldwin 1893
* "Odd D" #10003, an experimental electric locomotive.

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.

* E1
* E2
* E3
* E4
* E5
* E6
* E7
* E21
* E22
* E23
* E28

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.

* F1
* F2
* F3
* F21
* F22
* F23
* F24
* F25
* F26
* F27
* F30
* F31
* F61

G: 4-6-0

* G1
* G2
* G3
* G4
* G5 - Largest 10 wheeler built. PRR constructed 90 for use in commuter service and then built 30 for their subsidiary the Long Island RR.
* G53 - Owned By subsidiary Long Island RR

H: 2-8-0

* H1
* H2
* H3
* H4
* H5
* H6
* H8
* H9
* H10

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.

* I1s - heavy freight hauler.

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 250 2-10-4 "Texas" type locomotives to C&O plans; class J now being unoccupied, it was reused for them.

* J1 - 2-10-4 freight locomotives.
* J28 - experimental 2-6-2 locomotives.

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.

* K2 - 153 built at Altoona 1910-1911
* K3s - 30 built by Baldwin in 1913.
* K4s - 425 built by the PRR and Baldwin 1914-1928.
* K5 - 2 prototypes built

L: 2-8-2

Class L was assigned to the 2-8-2 "Mikado" type.

* L1s - freight twin to the famed K4s Pacific (575 built)
* L2s - the USRA standard light Mikado (5 built for PRR).
* L5 - PRR 2nd generation DC electric locomotive.
* L6 - PRR freight AC electric locomotives.

M: 4-8-2

* M1 - mixed-traffic Mountain type, latterly mostly used on fast freight.

N: 2-10-2

* N1s - Lines West heavy freight locomotive.
* N2s - USRA Standard 2-10-2, also used on Lines West.

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.

* O1 - 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.

* P5 - mixed-traffic electric locomotive.

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.

* Q1 - experimental duplex freight locomotive.
* Q2 - duplex freight locomotive.

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.

* R1 - experimental electric locomotive, surpassed by the GG1.

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 them. In the case of the S2, it was due to wartime limits on the use of advanced steel alloys.

* S1 - experimental duplex express passenger locomotive.
* S2 - experimental steam turbine locomotive.

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.

* T1 - Duplex express passenger locomotive.

Compound classifications

The PRR classified articulated locomotives and joined locomotive units by using multiples of the previous classifications.

Articulated steam:

* CC1s - 0-8-8-0 Mallet locomotive. Treated as two 0-8-0s for classification. One built.
* CC2s - 0-8-8-0 Mallet locomotive. 10 built.
* HC1s - 2-8-8-0 simple articulated locomotive. The PRR's only main-line articulated. One built.
* HH1s - 2-8-8-2 Mallet locomotive. Treated for classification purposes as two 2-8-0s back to back.
* HH1 - 2-8-8-2 Norfolk & Western class Y3 borrowed by PRR during World War II.

Articulated electric:

* AA1 - experimental 0-4-4-0 or B-B electric locomotives.
* BB1 - two-unit prototype AC electric 0-6-0+0-6-0 switching locomotive, later split into single units as class B1.
* BB2 - two-unit 0-6-0+0-6-0 DC electric switching locomotives, later split into single units as class B1.
* BB3 - two-unit 0-6-0+0-6-0 DC electric switching locomotives for LIRR, later split into single units as class B3.
* DD1 - two-unit DC electric locomotive, two 4-4-0 half-locomotives semi-permanently coupled back to back. Served between Manhattan Transfer and Penn Station, and to Sunnyside Yard.
* DD2 - experimental mixed-traffic AC electric locomotive.
* FF1 - 2-6-6-2 (1-C+C-1) experimental electric locomotive, 1917. Too powerful.
* FF2 - 2-6-6-2 motor-generator AC electric locomotives acquired second-hand from the Great Northern Railway (their classes Y1 and Y1a) in 1956.
* GG1 - express passenger and freight electric locomotive, highly successful.

Non-standard electric classification

* E2b - experimental B-B General Electric-built AC electric locomotives (3 pairs built).
* E3b - experimental B-B-B Baldwin-Lima-Hamilton-Westinghouse AC-DC rectifier locomotives (1 pair built).
* E3c - experimental C-C Baldwin-Lima-Hamilton-Westinghouse AC-DC rectifier locomotives (1 pair built)
* E44 - 1960 C-C General Electric AC-DC rectifier freight locomotives.

Early diesel classification

Later diesel classification