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 (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 type Steam Builder Baldwin Locomotive Works Serial number 60575, 60576, 60623-60625, 60666-60669 Build date August–October 1928
Specifications Configuration: ​  • Whyte 4-8-8-2 Gauge 4 ft 8 1⁄2 in (1,435 mm) standard gauge Driver dia. 63 in (1,600 mm) Adhesive weight 475,200 lb (215,500 kg; 215.5 t) Loco weight 614,600 lb (278,800 kg; 278.8 t) Boiler pressure 235 psi (1.62 MPa) Feedwater heater 4​1⁄4-BL Worthington Cylinder size 24 in × 32 in (610 mm × 813 mm) (bore × stroke)
Performance figures Tractive effort 112,760 lbf (501.6 kN), 116,900 lbf (520 kN) rebuilt
Career Operators Southern Pacific Railroad Class AC-4 Number in class 10 Numbers 4100 – 4109 First run October 1928 Disposition All 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.