In June 1937, the Pennsy acquired its first diesel locomotive: a 600-hp diesel-electric switch engine from Electro-Motive Corporation (EMC), a predecessor of General Motors Electro-Motive Division (EMD).
EMC called it an SW model; the railroad dubbed it class ES6.

The Pennsy bought its second diesel, another switcher, in October 1941: an EMD NW2 (Pennsy class ES10).
Wartime restrictions soon restricted locomotive builders' production of diesels intended for freight service.
Still, the Pennsy managed to add 15 Baldwin switchers and one EMD switcher.

After the war, the Pennsy began to buy diesel locomotives in earnest.
From 1945 through 1947, the railroad bought roughly 100 switcher, freight, and passenger diesels from various builders, then another 800 or so (total) in 1948 and 1949.

Passenger Units
For passenger units, the PRR purchased 60 E7-class locomotives from EMD, which the Pennsy dubbed the EP20 class.
46 of this number were designated "A" units, meaning that they had a cab for the train crew.
The remaining 14 were "B" units, cabless booster units controlled by an "A" unit.
The Pennsy also acquired 15 Alco PA1/PB1 units (class AFP20) and 27 Baldwin DR-6-4-2000 (class BP20) units.
The Baldwin units were originally for the passenger service fleet, but these locomotive proved troublesome and some were reclassified as BF16z freight locomotives.

The Pennsy also bought 24 Baldwin DR-12-8-1500/2s, called "Centipedes" and classified by Pennsy as BP60.
The Centipede had an immense weight of 593.71 short tons (538,600 kg) and a starting tractive effort of 205,000 pounds.
Unreliable and expensive to maintain, they were quickly relegated to helper service.

From 1950 to 1952, the Pennsy bought 74 EMD E8A locomotives, the successor to the E7, and classified them EP22s.
Also, the PRR purchased 40 dual-service EMD FP7A units which were similar to the E7/8 units, but using 4 axles instead of 6.

Freight Units
From the late 1940s through to the merger with the New York Central in 1968, the Pennsy purchased about 1,500 diesel freight units.
2/3 of these units were built by EMD, primarily F3s, F7s, GP9s, GP35s, SD40s, and SD45s.
Early on in dieselization of freight units, the Pennsy purchased cab units from Alco, Baldwin, EMD, and Fairbanks Morse.
Heading into the 1950s, the Pennsy continued to add more diesel units to their fleet.
Still preferring EMD, the Pennsy did add FA units and Baldwin RF-16 "Sharks".
In 1956, the Pennsy called for bids to supply a large order of diesel locomotives.
GM/EMD gave the Pennsy an exceptional deal on new, reliable GP9s, and received the entire order.
When this large diesel order arrived the following year, the Pennsy retired all of its remaining steam engines.
One of the losing bidders, Baldwin Locomotive Works — a longtime supplier of Pennsy locomotives — had been counting on winning at least some of the work.
When EMD won it all, the 126-year-old Baldwin declared bankruptcy.

For the last decade of Pennsy's existence (1960s), the Pennsy was ordering 4-axle power, buying Alco's 'Century' series locomotives, newcomer General Electric (GE) U25Bs, and EMD GP30s and GP35s.
But by 1965, the Pennsy turned exclusively to 6-axle power, buying Alco's C628 and C630, GE's U25C, U28C, and U30C, and EMD's SD35, SD40, and SD45.
In June 1937, the Pennsy acquired its first diesel locomotive: a 600-hp diesel-electric switch engine from Electro-Motive Corporation (EMC), a predecessor of General Motors Electro-Motive Division (EMD). EMC called it an SW model; the railroad dubbed it class ES6.
The Pennsy bought its second diesel, another switcher, in October 1941: an EMD NW2 (Pennsy class ES10). Wartime restrictions soon restricted locomotive builders' production of diesels intended for freight service. Still, the Pennsy managed to add 15 Baldwin switchers and one EMD switcher.
After the war, the Pennsy began to buy diesel locomotives in earnest. From 1945 through 1947, the railroad bought roughly 100 switcher, freight, and passenger diesels from various builders, then another 800 or so (total) in 1948 and 1949.
Passenger Units
For passenger units, the PRR purchased 60 E7-class locomotives from EMD, which the Pennsy dubbed the EP20 class. 46 of this number were designated "A" units, meaning that they had a cab for the train crew. The remaining 14 were "B" units, cabless booster units controlled by an "A" unit. The Pennsy also acquired 15 Alco PA1/PB1 units (class AFP20) and 27 Baldwin DR-6-4-2000 (class BP20) units. The Baldwin units were originally for the passenger service fleet, but these locomotive proved troublesome and some were reclassified as BF16z freight locomotives.
The Pennsy also bought 24 Baldwin DR-12-8-1500/2s, called "Centipedes" and classified by Pennsy as BP60. The Centipede had an immense weight of 593.71 short tons (538,600 kg) and a starting tractive effort of 205,000 pounds. Unreliable and expensive to maintain, they were quickly relegated to helper service.
From 1950 to 1952, the Pennsy bought 74 EMD E8A locomotives, the successor to the E7, and classified them EP22s. Also, the PRR purchased 40 dual-service EMD FP7A units which were similar to the E7/8 units, but using 4 axles instead of 6.
Freight Units
From the late 1940s through to the merger with the New York Central in 1968, the Pennsy purchased about 1,500 diesel freight units. 2/3 of these units were built by EMD, primarily F3s, F7s, GP9s, GP35s, SD40s, and SD45s. Early on in dieselization of freight units, the Pennsy purchased cab units from Alco, Baldwin, EMD, and Fairbanks Morse. Heading into the 1950s, the Pennsy continued to add more diesel units to their fleet. Still preferring EMD, the Pennsy did add FA units and Baldwin RF-16 "Sharks". In 1956, the Pennsy called for bids to supply a large order of diesel locomotives. GM/EMD gave the Pennsy an exceptional deal on new, reliable GP9s, and received the entire order. When this large diesel order arrived the following year, the Pennsy retired all of its remaining steam engines. One of the losing bidders, Baldwin Locomotive Works — a longtime supplier of Pennsy locomotives — had been counting on winning at least some of the work. When EMD won it all, the 126-year-old Baldwin declared bankruptcy.
For the last decade of Pennsy's existence (1960s), the Pennsy was ordering 4-axle power, buying Alco's 'Century' series locomotives, newcomer General Electric (GE) U25Bs, and EMD GP30s and GP35s. But by 1965, the Pennsy turned exclusively to 6-axle power, buying Alco's C628 and C630, GE's U25C, U28C, and U30C, and EMD's SD35, SD40, and SD45.
Pennsylvania Railroad