The Southern Pacific rostered 83 4-8-2 or Mountain type locomotives of class Mt, built in five groups between 1923 and 1930. These oil-fired locomotives were considered highly successful and were the mainstays of SP main-line passenger service until the advent of the GS-class 4-8-4s and then diesels. They weighed 368,000 pounds and developed a tractive force of 57,120 pounds. With 73-inch drivers, they had a boiler pressure of 210 p.s.i. and cylinder dimensions of 28x30 inches. (For other specifications see the commentary for No. 4324 following.) No. 4315, of class MT-1, was a member of the first group delivered in 1923-24 by the American Locomotive Company.

This image came from a long-discontinued Internet site, and also appears twice in the Railfan.net ABPR Archive posted by Bud Laws. The two postings have conflicting information; one gives the location as Berkeley, California, with a June 1946 date, and is attributed to Howard Davis of Pittsburgh, Pennsylvania. The other gives the date as June 1933; the earlier date is more likely correct since another source indicates that in 1939 No. 4315 received the characteristic skyline casing applied to all members of the Mt class, missing in this photo. Many members of the class also received a disc main driver, No. 4315 being fitted with hers in 1953.