It was Built in 1856 by William Mason of Taunton, Massachusetts, and is one of the only real American Standard's to operate as of today. (Yet, only half of the locomotive still has its original parts.)
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.
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.
William Mason has been preserved several times, as well as being owned by numerous different historical societies and museums.
It is now currently owned by the Baltimore and Ohio Railroad Museum in Balitmore, Maryland.
- This engine was featured in the film Wild Wild West as the "Wanderer" engine No 5. and The Great Locomotive Chase.
- There's a bit of debate over whether or not the William Mason is the olest operating steam locomotive in the US, yet its actually one of the oldest standard gauge locomotives to operate in partially-original condition.
- The refurbished and re-created parts used for William Mason are what help power the "ancient" steam locomotive.
- In 1998 The B&O "William Mason" as it arrived at the Strasburg Rail Road on February 26, 1998 after being out of service for approximately 36 years and later restored to service, and under steam.
- In The Great Locomotive Chase movie the engine plays "The General".
- 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.