Two wheels are connected by an axis and can rotate around this axis. The wheels are rolling on the x-y plane of the world frame. The coordinate system attached to the center of the wheel axis (frameMiddle) is constrained so that it is always parallel to the x-y plane. If all generalized coordinates are zero, frameMiddle is parallel to the world frame.
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});