Steam Locomotive Designer

Try designing your own steam locomotive. True steam locomotive design is an art (getting a locomotive that is not too slippery, one that will fit through clearances, having the correct horsepower for the terrain and loads it will have to pull, etc.). However, that doesn't prevent us from having some fun with a few of the important parameters that are used in steam locomotive design.

This Web page will allow you to specify the parameters that are used to determine Tractive Effort. As each parameter is changed, the resulting tractive effort and number of required axles will be computed and displayed.

Live Steam Modelers: While the sliders will not let you specify values in your ranges, you can still enter any value in the text fields. Remember to press the "Enter" key after changing a value.

[Java Steam Locomotive Designer Applet] Make sure Java is enabled on your browser.

NOTE: Press the <return> key after entering any text.

About Tractive Effort

Tractive effort is a theoretical value based upon the geometry of a locomotive and is only valid at a speed of 0 (zero). Tractive effort is sometimes incorrectly called "Tractive Power". Strictly speaking, power is work per unit of time and is often expressed in units of horsepower, watts, or joules per second. Tractive effort is simply a measure of the pulling force of a locomotive. The equation used for computing tractive effort includes a constant or coefficient. In the USA this coefficient was usually 0.85. Most sources describe this coefficient as the ratio of cylinder pressure to boiler pressure (15% loss). However a few sources (including Wikipedia) suggest this term also includes frictional losses. In advertising their roller bearings on steam locomotives, Timken chose to increase this coefficient to 1.0 to suggest that the use of their roller bearings resulted in no friction even though roller bearings had no affect on the pressure differential between the boiler and the cylinders. The use of roller bearings did, of course, increase drawbar pull because there was less friction to overcome. Both "tractive effort" and "drawbar pull" are quantities designed to describe the pulling ability of a locomotive. Both are measured in unit of force. As mentioned earlier, tractive effort is a theoretical value computed at a speed of 0. On the other hand, drawbar pull is a real, measured value that changes with speed and is measured by a dynamometer car.

Below are few examples of existing steam locomotive designs. Try entering one of these or one of your own.

Big Boy:
Boiler Pressure: 300 psi
Driver Diameter: 68 in
Cylinder Diameter: 23.75 in
Piston Stroke: 32 in
4 cylinders Tractive Effort: 135,375 lbs
Nickel Plate Berkshire:
Boiler Pressure: 245 psi
Driver Diameter: 69 in
Cylinder Diameter: 25 in
Piston Stroke: 34 in 2 cylinders Tractive Effort: 64,135 lbs
NYC J3a Hudson:
Boiler Pressure: 275 psi
Driver Diameter: 79 in
Cylinder Diameter: 22.5 in
Piston Stroke: 29 in 2 cylinders Tractive Effort: 43,440 lbs