Postcard photo of Southern Pacific locomotive 4100 and crew.

(EKC, Public domain, via Wikimedia Commons)


SP logo orange and yellow.


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).

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.


SP 4102 builders photo, circa 1935.

(State Government Photographer, CC0, via Wikimedia Commons)


Southern Pacific class AC-4 Overview

Type and origin
Power type: Steam
Builder: Baldwin Locomotive Works
Serial number: 60575, 60576, 60623-60625, 60666-60669
Build date: August–October 1928
Configuration: Whyte 4-8-8-2
Gauge: 4 ft 8+1⁄2 in (1,435 mm) standard gauge
Driver diameter: 63 in (1,600 mm)
Adhesive weight: 475,200 lb (215,500 kg; 215.5 t)
Locomotive weight: 614,600 lb (278,800 kg; 278.8 t)
Boiler pressure: 235 psi (1.62 MPa)
Feedwater heater: 41⁄4-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
Operators: Southern Pacific Railroad
Class: AC-4
Number in class: 10
Numbers: 4100 – 4109
First run: October 1928
Disposition: All scrapped