End view in use showing how the black tongue guides the wheels like a guard rail.
The comment under picture 1 on improving the operation is because where the black part is narrower a little further up, one wheel flange can actually roll on to the top of the rail. Then as the axle descends to this area the wheel is pulled over and drops into place with a clunk. That modification to the design eliminates this problem and I have since modified this unit by adding shims to the sides of the guide. (I have not posted new pictures because these shims are white plastic and don't show up on photos.)