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A standard set for one type of passenger coach wheels in the same bogie must not vary in diameter by more than 5 mm. Wheels under the same coach must not vary more than 10 mm on different bogies. The most modern vehicles might require a tolerance as low as 3 mm.
The very first trains only reached the speed of 23.6 mph (38 km/h). The average speed at which trains in the UK travel is between 60 and 65 mph (between 95 and 105 km/h). On main lines they can reach a speed of 130 mph (210 km/h). The actual maximum commercial train speed in the UK is 186 mph (300 km/h) and 249 mph (400 km/h) for Maglev trains.
An express train in India runs at an average speed of 50 kilometres an hour and a good train at 25 kilometres an hour only on broad gauge, EMUs (electrical multiple units) run at 40 kilometres an hour and ordinary passenger trains cover 36 kilometres an hour.
The fastest train on wheels is the French TGV150, and reaches a maximum speed of 355 mph (574 km/h). In 2027 Japan will introduce the L-Zero Maglev train, where they are already reaching test speeds of 362 mph (581 km/h). This is a train not on wheels, but on magnetic rails.