Article snapshot taken from Wikipedia with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
The first of these two, number 4000, entered service on May 26, 1909. It was rebuilt as a cab forward and reclassified as an MC-2 in June 1923. Another rebuild on June 4, 1931, "simpled" it with uniform cylinders and reclassified it as an AC-1. 4000 was scrapped on April 2, 1948. The second locomotive in this class, 4001, entered service on May 30, 1909. It was rebuilt as an MC-2 in April 1923, "simpled" on February 9, 1931, retired from active service on May 23, 1947, and scrapped on June 14, 1947, at SP's Sacramento shops.
References
Diebert, Timothy S. & Strapac, Joseph A. (1987). Southern Pacific Company Steam Locomotive Conpendium. Shade Tree Books. ISBN0-930742-12-5.