The roof was sloped down towards the back so that steam bubbles would flow forward till they reached the upper vertical part of the backplate which was in two pieces.

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.

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.

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.