Southern Pacific AC-1 2-8-8-2 4000 at Roseburg, Oregon, May 24, 1938. Photographer: Stan Kistler.

Built as a conventional Mallet Consolidation by Baldwin in April, 1909, one of two MC-1 class engines. They were an immediate failure on Donner Pass after nearly

asphyxiating the crew in the snowsheds and tunnels on the first trip. Baldwin was immediately telegraphed to change the design on the rest of the engines

being built and the cab forward was born. These first two MC-1's were rebuilt as cab forwards at Sacramento and reclassified MC-2.

(Craig Garver, Public domain,



Southern Pacific Railroad's AC-1 class of cab forward steam locomotives consisted of locomotives rebuilt from MC-1 and MC-2 class locomotives that were originally built by Baldwin Locomotive Works in 1909. The MC-2 class was the first class of locomotives built and delivered to SP as cab forward locomotives in late 1909. The AC-1 class was the first of the successful AC series of cab forward locomotives that numbered nearly 200 in total on the SP. Southern Pacific No. 4002 was rebuilt in June 1923 as a Cab Forward.

Their rebuilds into class AC-1 was around June 1931. SP used the rebuilt locomotives through the traffic rush of World War II, then the SP removed them from the roster soon after the war. They were all retired from active service by 1949 and were scrapped soon after their retirement. The last locomotive of this class that was scrapped was number 4014 on April 12, 1949. None of the original AC-1's were preserved. The only cab forward left in the United States of America is SP No. 4294, an AC-12 built in March 1944.


Southern Pacific No. 4002.

(NOKO, Public domain, via Wikimedia Commons)



Type and origin
Power type: Steam
Gauge: 4 ft 8+1⁄2 in (1,435 mm) standard gauge
Driver diameter: 57 in (1,448 mm)
Adhesive weight: 440,800 lb (199,900 kg)
Locomotive weight: 481,200 lb (218,300 kg)
Boiler pressure: 210 psi (1,400 kPa)
Feedwater heater: 41⁄4-BL Worthington
Cylinder size: 22 in × 30 in (559 mm × 762 mm) dia × stroke
Performance figures
Tractive effort: 90,940 lbf (404.5 kN)
Operators: Southern Pacific Railroad
Class: AC-1
Number in class: 15
Numbers: 4000 – 4010, 4012, 4014 – 4016
Retired: 1947 – 1948
Disposition: All scrapped


Schematic of 2-8-8-2 steam locomotive wheel arrangement. Front of locomotive on left.

(Gwernol, Public domain, via Wikimedia Commons)