The reference for locomotives and railcars
Southern Pacific class AC-9
United States | 1939 | 12 produced
No. 3802 in October 1939 in Kansas City
No. 3802 in October 1939 in Kansas City
Wesley Krambeck / collection Taylor Rush

While most of Southern Pacific's later articulated locomotives were Cab Forwards, the twelve AC-9s were of conventional design. Since there were no tunnels to pass through on the line from Tucumcari, New Mexico to El Paso, Texas, they were able to burn coal in the usual way. Since the coal available was not of high quality, the grate measured 139.5 square feet or just under 13 m².

These Yellowstones were the heaviest locomotives that the Southern Pacific operated. They had a skyline casing on the top of the boiler and were designed for speeds of 75 mph or 121 km/h. After their traditional line was converted to diesel, they were converted to run on oil and moved to other lines. They were retired between 1953 and 1956.

Axle config2-8-8-4 (Yellowstone (Mallet)) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Wheelbase66 ft 3 in
Fixed wheelbase16 ft 11 in
Total wheelbase113 ft
Service weight689,900 lbs
Adhesive weight531,200 lbs
Total weight1,090,600 lbs
Axle load66,400 lbs
Water capacity22,120 us gal
Fuel capacity56,000 lbs (coal)
Power sourcesteam
Estimated power6,000 hp (4,474 kW)
Optimal speed30 mph
Starting effort128,298 lbf
Power Plant
Driver diameter63.5 in
Boiler pressure260 psi
Expansion typesimple
Cylindersfour, 24 x 32 in
Grate area139.5 sq ft
Firebox area589 sq ft
Tube heating area6,329 sq ft
Evaporative heating area6,918 sq ft
Superheater area2,831 sq ft
Total heating area9,749 sq ft
Calculated Values
steam locomotive
last changed: 02/2024

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