Mastodon was the unofficial name of the Central Pacific Railroad's number 229, the world's first successful 4-8-0 steam locomotive.
The engine was designed and built by the road's master mechanic, Andrew Jackson "A.J" Stevens at Sacramento Locomotive Works in 1882. The locomotive had two highly unusual features: the firebox shape and the steam distribution. The only other locomotives to use such a valve gear were the 20 copies of Mastodon (see below) and CPR #237 El Gobernador. All other steam locomotives with slide valves had just a single valve in the center.
After being readied for its initial run, the engine had a minor accident where its large "balloon" stack was knocked off the smokebox. It was later discovered that the stack was nearly taller than the roundhouse doors. The problem was fixed and an impressive series of trials on the steep grades of the Sierra Nevada soon followed, in which it easily outperformed the smaller 4-4-0 and 4-6-0 engines used by the railroad at the time. Later, Mastodon was sent east to the Cooke Locomotive and Machine Works, along with blueprints and men who had built the engine, where more than 20 copies were produced; these were identical except for having their cylinder bore increased from to . The engine's success inspired railroad president Leland Stanford to instruct Stevens to build an even larger locomotive, which would be the largest in the world had at that time. This engine, a 4-10-0 named El Gobernador (CPRR #237), looked virtually identical to Mastodon with the exception of being longer and having an additional pair of driving wheels. However this engine, unlike its predecessor, was a failure and was scrapped in 1894.
Sometime in the early 20th century, No. 229 (now renumbered as Southern Pacific 2925) was converted from wood to oil-firing and was later assigned to the Oregon lines. Despite its historical significance, the engine was broken up for scrap at the Brooklyn Shops in Portland, Oregon, in June 1935 after a working life of 53 years.