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.
[citation needed] The diameter of the wheels also affects the maximum axle load of a Talgo RD wagon.
[citation needed] 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 as of 2008[update] was the highest axle load of any railway in the world.
especially on longer spans; in that case they require separate calculation of maximum allowable axle load.