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The ALCO PA was a family of A1A-A1A diesel locomotives built to haul . The locomotives 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. Designed by General Electric's Ray Patten (along with their ALCO FA cousins), they were of a cab unit design; both cab-equipped lead A unit PA and cabless booster B unit PB models were built. While externally the PB models were slightly shorter than the PA model, they shared many of the same characteristics, both aesthetically and mechanically. However, they were not as reliable as . ALCO's designation of P indicates that they were geared for higher speeds and passenger use, whereas the F designation marks these locomotives as being geared primarily for freight use. However, beyond this, their design was largely similar - aside from the PA/PB's both being larger A1A-A1A types with an even more striking nose - and many railroads used PA and FA locomotives for both freight and passenger service.
Although the majority of the PAs and PBs have been scrapped, six examples have survived. Four PAs are now preserved in railroad museums, while one PA has been restored to operational condition by Delaware-Lackawanna for use on excursion trains and a converted PB still remains in service as a power car.
The PAs, as well as their cousins, the , 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, numbered #51L, 51A and 51B, 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. This set was repowered in August, 1954 with EMD 16-567C engines rated at . This EMD repowering of the PAs was economically unfeasible, and the remaining Santa Fe PAs retained their 244 engines.
Four PA-1s previously operated by the Santa Fe were sold to Delaware and Hudson Railway in 1967. In 1974-1975, they were rebuilt for the D&H as PA-4s by Morrison Knudsen and equipped with ALCO's 251 V-12 engines.See Anderson, Norman E. and MacDermott, C. G., "PA4 Locomotive." (Burlingame, Chatham Publishing Co.)(1978). . Under D&H ownership, they were used by Amtrak for the Adirondack. (Amtrak itself only purchased EMD EMD E-unit and EMD F-unit from the railroads whose service it replaced for its diesel roster, and never owned any PAs.) They were used by the Massachusetts Bay Transit Authority in the late 1970s, then by Ferrocarriles Nacionales de México in 1978–81.
Fans deemed the PA one of the most beautiful diesels and an "Honorary Steam Locomotive", as noted by Professor George W. Hilton in a book review in September, 1968 Trains Magazine. When accelerating, until the turbocharger came up to speed, thick clouds of black smoke would pour from the exhaust stacks due to turbo lag. Photographing a moving PA while smoking became a prime objective of railfans.Ingles, J. David, Passenger Diesel Turned Legend, Trains Magazine January, 1997, p.54. “Honorary steam locomotive” at Trains Magazine''
| to New York Central Railroad 4212 and 4304 |
| Demonstrated on Canadian National, painted in CN green and gold, later to Missouri-Kansas-Texas Railroad as PA-2s 59A, C. Last PA-1s built. |
| First production PA1. To Gulf, Mobile and Ohio Railroad 292 |
| Four PA1s sold to Delaware & Hudson in 1967; became last to operate in U.S. |
| 59A, C were Alco PA-1 demonstrators rebuilt as PA-2s |
| 8011-8012 were originally owned by International & Great Northern |
| Unit 0783 to D&H in 1967 for parts. |
| To Southern Pacific Railroad 6067-6068 |
| Renumbered to 200-211, then to Southern Pacific 6055-6066 |
| 6005-6010A, C renumbered to 6005-6016, 6005-6010B renumbered to 5910-5915 |
| Last PA's built by ALCO |
| 607 converted for experimental coal-burning turbine in 1962 |
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