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 No. 25 was used extensively during the civil war to transport Union troops and supplies. B&O President John Garrett understood the financial and political benefits of siding with the Union, despite his personal ties to the South.
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 Baltimore, Maryland.
- The locomotive has also appeared in many movies, featuring in The Swan (1956), as The General in Disney's The Great Locomotive Chase (1956) and in the Civil War drama Raintree County (1957), as the "Wanderer" engine No. 5 in Wild, Wild West (1999), Tuck Everlasting (2002) and Gods and Generals (2003) and its bell make a brief appearance in Disney's Johnny Shiloh (1962).
- 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 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 refurbished and re-created parts used for William Mason are what help power the "ancient" steam locomotive.
- The B&O "William Mason" as it arrived at the Strasburg Railroad on February 26, 1998 after being out of service for approximately 36 years and later restored to service, and under steam.
- Then in June of 1998 it was back into operating condition on the Strasburg Railroad while hauling the regular train.
- 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 1865.
- As of 2015 William Mason has been in the process of undergoing an overhaul.
- It was renumbered 55 in 1882 and now it renumbered and named as 25 "William Mason".
- On November 20, 1950, it appeared as Milwaukee & Mississippi 1 for the Milwaukee Road Centennial.
- In April of 1999 it operate with deer horns on the headlight.
- The locomotive is sometimes on static display but it's still operates but only special occasions. (Just like V&T No. 22 (Inyo)).
- This locomotive was featured at the 1939 New York World's Fair.
- The museum operated the William Mason on select weekends in October until 2014 - after which the engine was taken out of service for its 1472-day inspection, which is still ongoing.
- During the inspection, it was found that the engine's firebox crown sheet would have to be replaced for the engine to continue to operate, the cost of which exceeded the budget allocated for the engine's restoration. 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. This livery, based on the layers of paint uncovered from the engine during restoration, is believed to be closer to that originally worn by the engine. The restored engine was returned to display in the roundhouse in September, 2017.
- 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.