[Bill_B]

As a point of reference for those interested who have not had the opportunity to see "under" a bridge, I will attempt to post a shot I made during disassembly of a bridge. You can see the ball-clearing mechanism has already been extracted. It is on the floor beyond the frame. This picture makes it clearer how complicated balls under the bridges can get. If under one of the joists, a ball has more impact on bridge movement than if it is between them.