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Pennsylvania Railroad's Odd D #10003 was an experimental electric locomotive built in 1907 by Baldwin and Westinghouse. It had a 4-4-0wheel arrangement in the Whyte notation, or 2-B in the AAR scheme. On the PRR, class D was assigned to 4-4-0 locomotives. Production classes of locomotive were assigned a number after the letter, but one-off locomotives were simply designated "Odd".
In testing, #10003 proved to be more stable at speed than the two class AA1 B-B locomotives the PRR had also constructed, so its 4-4-0 arrangement and high center of gravity was chosen as the design for the PRR's DD1 production locomotives.
Footnotes
^ Soloman, Brian (2003). Electric Locomotives. Saint Paul, MN: MBI Publishing. p. 44. ISBN978-0-7603-1359-6.
References
Staufer, Alvin F.; Pennypacker, Bert (1962). Pennsy Power: Steam and Electric Locomotives of the Pennsylvania Railroad, 1900-1957. Research by Martin Flattley. Carollton, Ohio: Alvin F. Staufer. p. 247. ISBN978-0-9445-1304-0.