Pennsylvania Railroad's Odd D #10003 was an experimental electric locomotive built in 1907 by Baldwin and Westinghouse. It had an 4-4-0 wheel 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.