An assembly joint for a wheelset rolling on the x-y plane of the world frame. The frames frame1 and frame2 are connected to rotating wheels; the frameMiddle moves in a plane parallel to the x-y plane of the world and should be connected to the vehicle body.
To work properly, the gravity acceleration vector g of the world must point in the negative z-axis, i.e.
inner Modelica.Mechanics.MultiBody.World world(n={0,0,-1});