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 rail tracks.
On railways, a given section of track is designed to support a maximum axle load. The maximum axle load is determined by weight of rails, density of sleepers and fixtures, train speeds, amount of ballast, and strength of bridges and earthworks. Because track and especially the rails, are expensive it is important to know axle load that a stretch of track can support. If the track is overloaded by trains that are too heavy, it can be damaged. Higher operating speeds can be achieved through reduced axle loads and increased density of sleepers.
The standard rail weight for British railways is now 113 lb/yd (56.1 kg/m). Before the 1990s, most diesel locomotives were built to a maximum axle load of 19 long tons (19.3 tonnes; 21.3 short tons) so the maximum locomotive weight was 76 long tons (77.2 tonnes; 85.1 short tons) for a four-axle locomotive and 114 long tons (115.8 tonnes; 127.7 short tons) for a six-axle one. Higher axle loads are now permitted, e.g. the Class 67 locomotive is a four-axle machine weighing 90 tonnes (88.6 long tons; 99.2 short tons), giving 22.5 tonnes (22.1 long tons; 24.8 short tons) on each axle.
The Fortescue Railway uses 68 kilograms per metre (140 lb/yd) rail on concrete sleepers and has a maximum axle load of 40 t (39 long tons; 44 short tons), which is the highest axle load of any railway in the world in 2016. In 2011, it was proposed to increase the axle load of the railway to 42 t (41 long tons; 46 short tons)
Bridges may have to carry several locomotives or wagons at the same time, especially for longer spans. They require separate calculation of maximum allowable axle load, and 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.
The term axle load is also applicable to trucks which is complicated by the fact that trucks may have more than two wheels per axle. In this case, 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.