thumb|PRR 5898 Herald

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.

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, similar in overall design and appearance to GG1.
  • 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).
  • E2c - 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.

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

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