Wheel-rail interface

The wheel-rail interface (also known as wheel-rail interaction) is the on-contact interaction between wheels and rails in railways. The term is used in connection with the design and management of their interaction. The focus is on safety improvement, maintenance and replacement costs reduction, downtime reduction, and energy and environmental management. The part of the Rail wheel that rests on the rail is known as the tread. The tread is held in place by a flange. Rail wheels can have either a single flange that rests only on one side of the rail or two flanges with one flange resting on each side of the rail.

...
Wikipedia