Continuous tractive effort

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The continuous tractive effort is the highest force that a vehicle can exert over an extended period of time. The term tractive effort is usually used in connection with trains and rail locomotives. Like all forces, tractive effort is most commonly measured in newtons.

For a locomotive to accelerate from a stationary position, it must apply a force to overcome the static friction of the train, and to accelerate it against its inertia. To do this, a particularly high tractive effort is required, usually the maximum tractive effort of the engine is applied. This means that the engine works to produce the highest possible force that it can exert onto the wheels to cause movement or motion.

Once the train is running at a constant velocity the train no longer needs to overcome its inertia to remain at the same velocity, and hence must only provide power to compensate for frictional forces; the tractive effort can hence be reduced to at or below the continuous tractive effort. Few engines can maintain work at the maximum tractive effort for very long in any case.

Each engine has a speed at which the tractive effort must be reduced from the maximum tractive effort to the continuous tractive effort or lower. Here is a table illustrating a selection of trains on the British rail network.

Class	Type	Top speed		Maximum tractive effort	Speed to reduce tractive effort	Continuous tractive effort	Maximum power at rail	Mass
Class	Type	mph	km/h	Maximum tractive effort	Speed to reduce tractive effort	Continuous tractive effort	Maximum power at rail	Mass
Class 08	Shunter	15		156 kN	8.8 mph	49 kN	194 kW	49.6 - 50.4 t
Class 33	Passenger	85		200 kN	17.5 mph	116 kN	906 kW	77.7 t
Class 56	Light freight	80		275 kN	16.8 mph	240 kN	1790 kW	125.2 t
Class 58	Light freight	80		275 kN	17.4 mph	240 kN	1780 kW	130 t
Class 59	Heavy freight	60 or 75		506 kN	14.3 mph	291 kN	1889 kW	121 t
Class 60	Heavy freight	60		500 kN	17.4 mph	336 kN	1800 kW	129-131 t
Class 66	Heavy freight	75		409 kN	15.9 mph	260 kN	1850 kW	126 t
Class 67	Light freight	125	200	141 kN	?? mph	90 kN	1860 kW	90 t

In general, it is more common for heavy freight trains (such as Class 59, Class 60 and Class 66 locomotives) to have a high continuous tractive effort due to the mass which they haul. Light freight trains (such as Class 56, Class 58 and Class 67 locomotives) and passenger trains (such as Class 33 and Class 43 / Intercity High Speed Train locomotives) usually have much lower maximum tractive efforts because the trains are much lighter and therefore there is much less resistance acting upon the train in terms of friction.