Axle load

The axle load of a wheeled vehicle is the total weight bearing on the roadway for all wheels connected to a given axle. Axle load is an important design consideration in the engineering of roadways and railways, as both are designed to tolerate a maximum weight-per-axle (axle load); exceeding the maximum rated axle load will cause damage to the roadway or railway tracks.

Railway use
On railways, a given section of tracks is designed to support a maximum axle load. The maximum axle load is determined by train speeds, weight of rails, density of sleepers and fixtures, amount and standard of ballast, and strength of bridges and earthworks. Higher operating speeds can be achieved by reducing axle loads and increased load-carrying capacity. Operating above the specified load can cause catastrophic failure of track components. The diameter of the wheels also affects the maximum axle load of a Talgo RD wagon.

United Kingdom
The standard rail weight for British railways is now 113 lb/yd. Before the 1990s, most diesel locomotives were built to a maximum axle load of 19 LT so the maximum locomotive weight was 76 LT for a four-axle locomotive and 114 LT for a six-axle one. Higher axle loads are now permitted, e.g. the Class 67 locomotive is a four-axle machine weighing 90 t, giving 22.5 t on each axle.

Australia
The Fortescue railway uses 68 kg/m rail on concrete sleepers and has a maximum axle load of 40 t, which was the highest axle load of any railway in the world. In 2011, it was proposed to increase the axle load of the railway to 42 t.

Kenya
In 2022, sixteen new metre-gauge locomotives were supplied by CRRC with axleloads of 12.5 tonnes, 14 tonnes, and 18 tonnes respectively.

Bridge loading
Bridges may have to carry several locomotives or wagons at the same time. especially on longer spans; in that case they require separate calculation of maximum allowable axle load. A weak bridge may limit the axle load of the full line. Theodore Cooper developed the E10 loading system for calculating the strength of bridges.

Roadway use
The term axle load is also applicable to trucks, and this context is made more complex by some trucks having more than two wheels per axle. The axle load remains the same, but the load borne by the individual wheels is reduced by having more contact area (more wheels, larger tires, lower tire pressure) to distribute the load.