The Southern Pacific was one of the railroads which used the Harriman Common Standard Pacific as a light express locomotive. These had drivers of 77 inches and cylinders of 22 by 28 inches. The valve gear was of the Stephenson type and mounted between the frames. The first five were built by Schenectady in 1904, numbered 2400 to 2404 and designated class P-1. 23 more followed from Baldwin in 1906 and 1907 which were numbered 2405 to 2427 and assigned to the same class. Later locomotives were virtually identical, but designated P-3. 2428 to 2437 were built by Baldwin in 1911 and 2459 and 2460 were built in the Southern Pacific's own shops in 1917 and 1918.

When they were superheated, the 245 2¼-inch tubes were replaced against 182 two-inch tubes and 24 5⅜-inch flues. At the same time, the boiler pressure was increased from 200 to 210 psi and the dimensions of the cylinders kept unchanged. Only eight class members were rebuilt on a larger scale between 1927 and 1929. Now being designated class P-4, these had a cylinder diameter of 23 inches, outside Walschaerts valve gear, a feed water heater and a trailing truck booster.