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Tractive effort is the force that a locomotive or other motive power (such as a railcar or multiple unit) applies to hauling its load, including the weight of that vehicle itself. It is not to be confused with power, but depends on a combination of that, the weight of the driving wheels on the rails called adhesive weight, the friction between those wheels and the rails, the speed of the vehicle, the gradient of the line, etcetera.
Generally speaking, the same tractive effort will pull a lighter load going uphill than on the level. Downhill, braking might be needed instead. The effort required to stop within a safe distance is called the braking effort.
The Festiniog Railway was laid out to use gravity for the loaded slate trains going downhill from Blaenau Ffestiniog to the Cob, where horses took over for the last bit to Porthmadog. Nowadays that gradient is interrupted because of the Deviation.
Notes and References
- "Driving wheels" in this connection means all wheels that transfer power from the vehicle to the rails. On a locomotive with coupling rods, like most steam locomotives and older diesels, that means both the drivers in the narrow sense – the wheels that are driven directly by the cylinders or jackshaft – and the wheels that are coupled to them.