First add some Reference planes centered on the wheel and the rail. Then apply a Tangent constraint to one face of the wheel and the mating face of the rail. Add another Tangent constraint to the opposite wheel face and rail face. This will mate the wheel to the rail but you won't be able to move the wheel along the rail--if that is your intent.
In order to move the wheel along the rail, suppress or delete one of the tangent constraints and apply a coincident constraint between the two reference planes. This is one time you will have to turn off Minimum Motion in order to drag the wheel along the rail.