Although the Baltimore and Ohio had procured a locomotive with a 4-8-0 wheel arrangement in 1855 with the “Centipede”, the “Mastodon” was the first successful locomotive with this wheel arrangement. Completed in 1882, it was the heaviest locomotive in the world at the time. Shortly after its roll-out, test runs were undertaken in the Sierra Nevada with the locomotive number 229, during which, as expected, it performed significantly better than the existing Americans and Ten-Wheelers. When the operation was taken over by the Southern Pacific, it was renumbered 2925.

The shape of the boiler had some special features, including a firebox with a lower ceiling in the area of the driver's cab, which was around 13 feet long. The cylinders each had four slide valves, two of which were operated by one valve gear each. This type of cylinders was also inherited by “El Gobernator”, the even larger locomotive with 4-10-0 wheel arrangement.

Built by the Central Pacific workshops, number 229 was later sent to the Cooke Locomotive and Machine Works in New Jersey to set up series production. Depending on the source, 20 or 25 other machines were made, which had slightly larger cylinders. The first locomotive was later modernized and converted to oil firing, which probably also applies to the production locomotives. They were scrapped in June 1935.