William Mason is a 4-4-0 steam locomotive currently on display at the Baltimore and Ohio Railroad Museum in Baltimore, Maryland, United States. It was built for the Baltimore and Ohio Railroad, carrying that railroad's number 25. The locomotive is named in honor of its builder, William Mason, who built around 754 steam locomotives at his Mason Machine Works firm in Taunton, Massachusetts, from 1853 until his death in 1883. The engine had been one of the oldest operable examples of the American Standard design, and is the fourth oldest Baltimore and Ohio locomotive in existence, the oldest being the 0-4-0 no. 2, the Andrew Jackson from 1836, second oldest is the no. 8 0-4-0, John Hancock built later that same year, and the third being the 0-8-0 no. 57, Memnon of 1848 (The preserved Tom Thum