The reference for locomotives and railcars
Southern Pacific classes AC-7, AC-8 and AC-10 to AC-12
United States | 1937 | 143 produced
AC-7 No. 4154 shortly after its delivery in 1937
AC-7 No. 4154 shortly after its delivery in 1937
R.J. Foster / collection Taylor Rush
AC-11 No. 4269. Note the staff member on the “Monkey Deck”
AC-11 No. 4269. Note the staff member on the “Monkey Deck”
collection Taylor Rush

After the AC-6 class cab forwards, 26 slightly larger AC-7 class machines were built in 1937. They were technically almost identical to their predecessors and differed visually in that they had a larger tender and a cab that was beveled at the front. The two frames of the locomotives were now cast together with the cylinders in one piece each. These were followed by 27 almost identical AC-8s, 40 AC-10s, 30 AC-11s and 20 AC-12s. The last AC-12 was also the last steam locomotive to be built for the Southern Pacific. These five subclasses were retired between 1954 and 1959.

VariantAC-7, AC-8AC-10 to AC-12
Built1937, 19391942-1944
Axle config4-8-8-2 (Mountain Mallet) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Wheelbase66 ft 10 in67 ft 3 in
Fixed wheelbase16 ft 10 in
Total wheelbase112 ft 2 in
Service weight639,700 lbs657,900 lbs
Adhesive weight514,800 lbs531,700 lbs
Total weight934,700 lbs977,900 lbs
Axle load67,000 lbs69,100 lbs
Water capacity22,000 us gal
Fuel capacity6,400 us gal (oil)6,100 us gal (oil)
Power sourcesteam
Indicated power6,000 hp (4,474 kW)
Optimal speed31 mph
Starting effort123,364 lbf
Power Plant
Driver diameter63.5 in
Boiler pressure250 psi
Expansion typesimple
Cylindersfour, 24 x 32 in
Grate area139 sq ft
Firebox area513 sq ft478 sq ft
Tube heating area5,992 sq ft
Evaporative heating area6,505 sq ft6,470 sq ft
Superheater area2,988 sq ft2,615 sq ft
Total heating area9,493 sq ft9,085 sq ft
Calculated Values
steam locomotive
cab forward
last changed: 03/2023

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