Wheelset (rail transport)

A wheelset is a pair of railroad vehicle wheels mounted rigidly on an axle such that both wheels rotate in unison. Wheelsets are often mounted in a bogie ("truck" in North America) – a pivoted frame assembly holding at least two wheelsets – at each end of the vehicle. Most modern freight cars and passenger cars have bogies each with two wheelsets, but three wheelsets (or more) are used in bogies of freight cars that carry heavy loads, and three-wheelset bogies are under some passenger cars. Four-wheeled goods wagons that were once near-universal in Europe and Great Britain and their colonies have only two wheelsets; in recent decades such vehicles have become less common as trainloads have become heavier.