A wheel unit is disclosed for supporting a structure for movement on a track. The unit includes a rotatable wheel having a substantially flat bearing surface and supported on an axis for running on a track. A housing supports the wheel and includes a convex-shaped guide for maintaining the wheel on the track. A wheel and track combination is also disclosed wherein the wheel has a contact surface having a first dimension and supported by the housing to rotate about an axis. The track supports the housing and wheel and includes a contact surface for contacting the contact surface of the wheel and wherein the contact surface of the track has a second dimension larger than the first dimension of the contact surface on the wheel. Preferably, a guide keeps the wheel on the track while permitting the wheel to move laterally on the track.