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ALCO PA refers to a family of A1A-A1A diesel locomotives built to haul passenger trains that were built in Schenectady, New York in the United States by a partnership of the American Locomotive Company (ALCO) and General Electric (GE) between June, 1946 and December, 1953. They were of a cab unit design, and both cab-equipped lead A unit PA and cabless booster B unit PB models were built.

The PAs, as well as their cousins, the ALCO FAs, were born as a result of Alco's development of a new diesel engine design, the Model 244. In early 1944, development started on the new design, and by November 1945, the first engines were beginning to undergo tests. This unusually short testing sequence was brought about by the decision of Alco's senior management that the engine and an associated line of road locomotives had to be introduced no later than the end of 1946. In preparation for this deadline, by January 1946, the first 16 cylinder 244 engines were being tested, and while a strike delayed work on the locomotives, the first two PA units were released for road tests in June 1946, for testing for one month on the Lehigh Valley Railroad. After these first tests were completed, the locomotives returned to the factory for refurbishment and engine replacement. In September 1946, the first production units, an A-B-A set of PA1s in Santa Fe colors were released from the factory, and sent to New York's Waldorf-Astoria Hotel, which had a private railroad siding, for exhibition before being launched into road service.

The ALCO 244 V16 diesel prime mover proved to be the undoing of the PA: The engine had been rushed into production, and proved to be unreliable. The PA locomotives failed to capture a marketplace dominated by General Motors Electro-Motive Division and their E-units. The original Santa Fe three unit set #51L, 51A and 51B was repowered in August 1954 with EMD 16-567C engines rated at 1,750 hp. This EMD repowering of the PAs was economically unfeasible and the remaining Santa Fe PAs retained their 244 engines. The later 251-series engine, a vastly improved prime mover, was not available in time for ALCO to recover the loss of reputation caused by the unreliability of the 244. By the time the ALCO 251 engine was accepted into widespread use, General Electric (which ended the partnership with ALCO in 1953) had fielded their entries into the diesel-electric locomotive market. General Electric eventually supplanted ALCO as a manufacturer of locomotives. ALCO's loss of market share led to its demise in 1969.