@standupmaths imagine a truck with an upper section and lower section, with supports on the front and back holding up the upper section, but otherwise a big ol' empty gap between. The front and back supports are strong enough to hold the entire upper section on their own. As the front of the truck approaches the bridge, the front support detaches and lowers allowing the bridge to go through the truck. Once the front has cleared the bridge, the front support is re-engaged, and the back support detaches allowing the bridge to pass cleanly through. Now the truck can far exceed the bridge height, just has to be longer than the bridge width.