The reference for locomotives and railcars
Southern Pacific classes AC-4, AC-5 and AC-6
United States | 1928 | 51 produced
AC-6 No. 4133 on a works photo
AC-6 No. 4133 on a works photo

After the Southern Pacific had already converted several 2-8-8-2 Mallets into Cab Forwards, the 1928 AC-4 was the first class to be built as a cab forward from the start. Superintendent George McCormick was responsible and production took place at Baldwin. Like the AC-1 to AC-3, the AC-4 also had four cylinders of the same size without compound working. Thanks to a two-axle leading bogie, a larger firebox could now be accommodated. The delivery of oil to the front of the locomotive was ensured using the proven system with an overpressure of 5 psi

The AC-4's valve gear had a limited cutoff, which initially reduced the starting tractive effort somewhat and was later removed. After ten AC-4s, 16 AC-5s were built, which were slightly heavier. The 25 AC-6s, which also had a higher boiler pressure and a smaller superheater, were once again significantly heavier. They received a new, cast frame in 1947 and 1948, which further increased the weight. All three classes were retired between 1953 and 1956.

VariantAC-4, AC-5AC-5AC-6
Axle config4-8-8-2 (Mikado Mallet) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length loco77 ft 11 1/4 in
Wheelbase66 ft 10 in67 ft 3 in
Fixed wheelbase16 ft 11 in
Total wheelbase106 ft 5 in106 ft 11 1/2 in
Service weight614,600 lbs622,600 lbs639,500 lbs
Adhesive weight475,200 lbs482,500 lbs517,000 lbs
Total weight875,600 lbs883,600 lbs900,500 lbs
Axle load60,500 lbs61,900 lbs69,100 lbs
Water capacity16,152 us gal
Engine output5,640 hp (4,206 kW)6,000 hp (4,474 kW)
Power Plant
Driver diameter63.5 in
Boiler pressure235 psi250 psi
Cylindersfour, 24 x 32 in
Grate area139 sq ft
Firebox area513 sq ft
Tube heating area6,507 sq ft5,959 sq ft
Evaporative heating area7,020 sq ft6,472 sq ft
Superheater area2,989 sq ft2,736 sq ft
Total heating area10,009 sq ft9,208 sq ft
Calculated Values
steam locomotive
cab forward
last changed: 12/2023

We use cookies to save the following settings:

  • selected navigation structure
  • selected language
  • preferred units
  • spelling of railway company names

If you refuse the use of cookies, the settings will only be retained for the current session and will be reset to the default values the next time you visit the site.

Display of units

Here you can set the desired unit system for the technical data.

  • Metric: Lengths in meters, weights in tonnes, and volumes in cubic meters
  • Imperial (UK): Lengths in feet/inches, weights in long tons and volumes in imperial gallons
  • Imperial (US): Lengths in feet/inches, weights in pounds, and volumes in US gallons
  • Individual: Depends on the country of origin of each locomotive
Operator names

Here you can set the display of railway company names.

  • Short: Abbreviation or short form of the name
  • Standard: commonly used name, partially translated to English
  • Complete: full name in local language