The axle load of a wheeled vehicle is the total weight felt by the roadway for all wheels connected to a given axle. Viewed another way, it is the fraction of total vehicle weight resting on 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.
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On railways, a given section of track is designed to a maximum axle load. The maximum axle load is related to the strength of the track, which is determined by weight of rails, density of sleepers and fixtures, train speeds, amount of ballast, and strength of bridges. Because track and especially the rails are expensive, it is desirable to optimise the track for a given axle load. If the track is overloaded by trains that are too heavy, it can be destroyed in a short time. It is convenient for the steelworks that rails are made in a limited number of sizes, so that a perfect match of rail weight and axle load is rarely achieved. New rail is often reserved for heavy main line use, which releases good but lighter rail that can be cascaded for lighter duties on branch lines. The lightest rail cascaded from the lightest branch lines may have no railway use other that for structural items such as fenceposts, telegraph posts and for reinforcing concrete. Increase density of sleepers and reduce axle load can help to increase train speeds.
Light railways were built with rail weighing as little as 30 lb/yard but main lines used much heavier rail. On former British Rail lines the rail is mostly 90 lb/yard or 120 lb/yard.
In British Rail days most diesel locomotives were built to a maximum axle load of 19 long tons (19.3 metric tonnes, 21.28 short tons) so the maximum locomotive weight was 76 long tons (77.2 metric tonnes, 85.2 short tons) for a four-axle locomotive and 114 long tons (115.8 metric tonnes, 127.68 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 metric tonnes, giving 22.5 metric tonnes on each axle.
The new branch line from Blackwater, Queensland to Rolleston is being built in 2005 to haul the following:
The new heavy-duty Fortescue Railway:
The absolute maximum axleload for railways is about 40t, above which the rails start to be pulverised by the pasage of trains.
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 felt by the individual wheels is reduced by having more wheels to distribute the load.