The Pennsylvania Railroad's steam locomotive class D1 (formerly Class A, pre-1895) comprised thirteen 4-4-0 locomotives for express passenger service, constructed at the railroad's own Altoona Works (now owned by Norfolk Southern) during 1868–1872.

[3] They were the first standardized class of locomotives on the railroad and shared many parts with other standard classes.

[1] The PRR was the first American railroad to adopt the Westinghouse air brake, the first tests of which were made in September 1869; Class A locomotives were among those fitted with air brake equipment for those earliest tests.

[4][5] They remained in service until 1945, and were all withdrawn and scrapped by 1946.

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