Most modern depots are equipped with a wheel profiling facility known as a wheel lathe. These are normally designed so that the wheels can be reprofiled while still on the train. Removing the wheels requires the train to be lifted and this is an expensive business and very time-consuming. To avoid this, the underfloor wheel lathe or "ground" wheel lathe was developed like the one shown in the photo left.
Wheels can be removed from a train by a "wheel drop", where the wheelset is lowered underneath the train into a basement below the depot floor. Sometimes, whole toolrooms are provided in such areas but the ground conditions sometimes make such places difficult to keep dry and difficult to conform with modern evacuation requirements.Modern wheel lathes can also reprofile a wheelset which has been removed from the train. Otherwise a separate wheel turning facility has to be provided in the workshop. Cutting has been the most common method of reprofiling but, recently milling machines, have been making a comeback as they can offer a longer tool life and better tolerance control on diameters.
Train wheels wear just as car tyres do and they need to be checked regularly. When the wear reaches certain limits, the treads either have to be reprofiled to the correct shape or the wheels replaced. Reprofiling wheels is a slow and expensive process but train and wheel design and maintenance has improved considerably over recent years, reducing the periods between visits for reprofiling. Even so, there are still persistent cases of railways running into unforeseen or unusual wheel wear problems and the wheel/rail interface still needs a lot more research before it is fully understood.
Wheels on a bogie or wheels on a single vehicle must be reprofiled within limits compared with each other. For example, a standard set for one type of passenger coach says that 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. When wheels which drive a speedometer are reprofiled, the speedometer will have to be adjusted to compensate for the difference in wheel diameter caused by the reprofiling.
Some modern wheels lathes are designed to turn both wheelsets on a bogie at the same time. These "double-headed" lathes have developed as a result of electronically controlled AC motors, which require that the motors in the same circuit turn at the same speed so as to match the inverter frequency. This makes it essential that wheel diameters with motors within a traction power circuit are equal.
Although it might seem obvious, the roundness of wheels is important, especially at very high speeds. An eccentric wheel can cause extreme loads on the wheel, axle, bearing and suspension, leading to failures. An "unround" (out of round) or eccentric wheel is alleged to have led to wheel tyre failure of a German ICE at Eschede in 1998, causing a high-speed crash with heavy loss of life. The wheel is alleged to have had an eccentricity (difference between major and minor axes of the ellipse) of 1.1mm, against a limit of 0.6mm. Wheels are often damaged by skidding during braking. Skidding (called sliding) causes a flat patch (called a "flat") to wear on the tyre and, when the wheel begins rolling again after a slide, the familiar "tap, tap, tap..." of the flat will be heard. Overheating during braking can also damage a wheel, as shown in the next photo:
Even if wheels, by some lucky combination of circumstances, do not wear significantly, reprofiling to remove work-hardened metal is likely to be needed at around 1 million km, otherwise Martensite fragments can drop out of the wheel tread, leading to the type of damage shown in the photo above. This damage can also be caused by local overheating during skidding and/or braking.