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Southern Pacific AC-4 No. 4100 and crew.
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Southern Pacific AC-4 No. 4100 and crew.

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Southern Pacific Class AC-4 Articulated Consolidation Cab Forward Steam Locomotive
Southern Pacific Railroad's AC-4 class of steam locomotives were the first class of 4-8-8-2 cab forward locomotives. They were intended to improve on the railroad's MC class 2-8-8-2 locomotives with a larger firebox.

The AC-4s were the first SP Mallets built for simple expansion. Baldwin Locomotive Works built them in August through October of 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.

Southern Pacific class AC-4 Overview
Type and origin
Power type: Steam
Builder: Baldwin Locomotive Works
Serial numbers: 60575, 60576, 60623-60625, 60666-60669
Build dates: August–October 1928
Configuration: 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)
Loco weight: 614,600 lb (278,800 kg; 278.8 t)
Boiler pressure: 235 psi (1.62 MPa)
Feedwater heater: 4¼-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

Operator: Southern Pacific Railroad
Class: AC-4
Number in class: 10
Numbers: 4100 – 4109
First run: October 1928
Disposition: scrapped

See also:

Cab Forward Locomotives

4-8-8-2 Locomotives

Steam Locomotives

Baldwin Locomotive Works