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 Thumb and Lafayette engines are replicas built by the road for exhibition purposes in 1926 and 1927, respectively). While operable, William Mason had been one of the oldest operational locomotive in the world, and the oldest in the western hemisphere. History and career The engine was built by William Mason for the Baltimore and Ohio in 1856, most likely for passenger service. The engine, while not given a name (the road had ended the practice of naming locomotives at the time no. 25 was built), was the road's second engine to be numbered 25, replacing an earlier 4-4-0 of that number built by William Norris in 1839. Among the engine's notable features is the "three-point suspension," where unlike most earlier locomotives (i.e. the road's Lafayette), which the front bogie has its wheels closely spaced, the number 25's bogie had its wheels spread apart, with the cylinder mounted horizontally between them. While the engine was not the first to have this design, it represented a major improvement in locomotive design which would come to define the "American Standard" locomotive. Another, similarly revolutionary design was the engine's smokebox. Unlike earlier designs, such as that of the General, built a year earlier, the number 25 had its smokebox sitting on a "saddle" which carried the cylinders. This design further lowered the engine's center of gravity and made re-boilering easier. The number 25 was the road's first engine to have this smokebox design, as well as the road's first engine to have Stephenson link motion valve gear. Retirement and exhibition The number 25 served on the Baltimore and Ohio