Southern Pacific class AC-4

Southern Pacific Railroad's AC-4 (meaning Articulated Consolidation) class of steam locomotives was the first class of 4-8-8-2 cab forward locomotives. They were intended to improve on the railroad's MC (meaning Mallet-Consolidation) class 2-8-8-2 locomotives with a larger firebox. Hence the four-wheel leading truck (instead of the two-wheel).

Southern Pacific class AC-4
Southern Pacific 4100 and its crew
Type and origin
Power typeSteam
BuilderBaldwin Locomotive Works
Serial number60575, 60576, 60623-60625, 60666-60669
Build dateAugust–October 1928
Gauge4 ft 8 12 in (1,435 mm) standard gauge
Driver dia.63 in (1,600 mm)
Adhesive weight475,200 lb (215,500 kg; 215.5 t)
Loco weight614,600 lb (278,800 kg; 278.8 t)
Boiler pressure235 psi (1.62 MPa)
Feedwater heater4¼-BL Worthington
Cylinder size24 in × 32 in (610 mm × 813 mm) (bore × stroke)
Performance figures
Tractive effort112,760 lbf (501.6 kN), 116,900 lbf (520 kN) rebuilt
OperatorsSouthern Pacific Railroad
Number in class10
Numbers4100 4109
First runOctober 1928
DispositionAll scrapped

The AC-4s were the first SP Mallets built for simple expansion. Baldwin Locomotive Works built them in August through October 1928 with a maximum cutoff of 70%, so tractive effort was rated at 112,760 lbf (501.6 kN); a few years later limited cutoff was dropped and calculated tractive effort increased to 116,000 lbf (520 kN).

The AC-4s were removed from service starting in 1953 and all ten were scrapped by June 1955.


  • 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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