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).

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.

The locomotive is one of the engines that pulled the train which carried Abraham Lincoln from Springfield, Illinois, to Washington, D.C., for his inauguration in 1861.

Fortunately, the Baltimore and Ohio had greatly favored its locomotives, and preserved its best examples, including number 25.

It had remained operable during the 1950s, when it was used in movies, most notably The Great Locomotive Chase in 1956, and Raintree County a year later.

In February 2003, the roof of the Baltimore and Ohio Railroad Museum's roundhouse, where the William Mason was usually displayed, had collapsed from a major snowstorm.

The museum operated the William Mason on select weekends in October until 2014 - after which the engine was taken out of service for its 1,472-day inspection.

[4][5] Thus, the museum instead opted for a cosmetic restoration, with the engine receiving a new livery of green (a different shade from that worn post-1999) with red and gold lining.