Scroll down below the documentation table for a detail picture and its explanation.
The light wood bars at the right and left are called a French Cleat; they draw the bridge in tight as it's lowered, thereby ensuring steadiness and proper spacing of the rails on both mainlines. A brass pin on the bridge slips into the brass socket in the middle to ensure proper alignment. The black thing in the middle is a microswitch that cuts power to the two adjacent modules when the bridge is raised.