Trochoid

A wheel rolls along a straight line carrying a point at some distance from its centre, and the point draws a trochoid, from the Greek trokhos, a wheel. With the point inside the rim the curve is close to a sine wave, and on the rim it is the cycloid. Taken outside the rim, the point draws loops, which is why part of a moving train, the bottom of each wheel’s flange, travels backward.

x = aθ - b sin θ, y = a - b cos θ

On mathcurve.com as Trochoïde