A 3-wheeled carriage moves along a rail consisting of straight, convex, concave and transitional portions.
To maintain a continuous gapless contact between the wheels and the rail, the widths of three portions are not the same.
They are larger at convex portions, smaller at concave portions in comparison with the widths of straight portions. The differences depend on wheel diameter, radii of convex and concave portions.
The transitional portions where the wheels may lose the contact should be very short.