Wheelset (rail)

A wheelset is the wheel - axle assembly of a railroad car. The frame assembly beneath each end of a car, railcar or locomotive that holds the wheelsets is called the bogie (or truck in North America). Most North American freight cars have two bogies with two or three wheelsets, depending on the type of car; short freight cars generally have no bogies but instead have two wheelsets.

Two-axle cars operating on lines with sharp curves, such as Queensland Railways, used Grovers bogies.

Rubber-tyred metros feature special wheelsets with rubber tyres outside of the special flanged steel wheels. The unusually large flanges on the steel wheels guide the bogie through standard railroad switches and in addition keep the train from derailing in case the tires deflate.

Most train wheels have a semi-conical taper of about 1 in 20. The semi-conical shape helps steer the wheel set around curves, so that the wheel flanges do not come in contact with the rail sides. The rails generally slant inwards at the same rate as the wheel conicity. As the wheels approach a curve, they tend to continue in a straight path due to the inertia of the rail car. This inertia makes the wheel set to shift sideways as the track curves under it. Due to this fact, the effective diameter of the outer wheels needs to be greater than that of the inner ones. Since the wheels are joined rigidly by the axle, the outer wheels travel farther, then causing the train to naturally follow the curve. For more information on this process, see Hunting oscillation.

...
Wikipedia