Southern Pacific class AC-6

Southern Pacific Railroad's AC-6 class of steam locomotives was the third of the railroad's classes built with a 4-8-8-2 wheel arrangement. Like the earlier AC-4 and AC-5 classes, the AC-6 class were cab forward locomotives. The AC-6 was slightly larger than the previous classes with a higher boiler pressure and tractive effort rating.

Southern Pacific class AC-6
Type and origin
Power typeSteam
BuilderBaldwin Locomotive Works
Serial number61353, 61354, 61382-61389, 61416, 61426-61429, 61491-61494, 61535-61538, 61543, 61544
Build dateMay–December 1930
Gauge4 ft 8 12 in (1,435 mm) standard gauge
Driver dia.63 in (1,600 mm)
Adhesive weight517,000 lb (235,000 kg; 235 t)
Loco weight639,500 lb (290,100 kg; 290.1 t)
Boiler pressure250 psi (1.7 MPa)
Feedwater heater6S Worthington
Cylinder size24 in × 34 in (610 mm × 864 mm) (bore × stroke)
Performance figures
Tractive effort124,300 lbf (553 kN)
OperatorsSouthern Pacific Railroad
Number in class25
Numbers4126 4150
First runJuly 10, 1930
Retired1953 1955

In 1947 and 1948, this class was rebuilt with cast steel frames, increasing the overall weight to 648,000 lb (with 523,600 lb on the drivers). At this time, most of the locomotives in the class were also equipped with larger cab windows that became standard for SP's cab forward locomotives beginning with the AC-8 class. One of those locomotives that had been turned from AC-6 to AC-8 was the number 4146.

This class was removed from active service between 1954 and 1955 and they were all scrapped by March 1956.


  • Diebert, Timothy S. & Strapac, Joseph A. (1987). Southern Pacific Company Steam Locomotive Conpendium. Shade Tree Books. ISBN 0-930742-12-5.
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