The design of this bridge is inspired by necessity; I needed to design a bridge that used only one motor. I did not want to worry about synchronizing motors at each end of the bridge, and...well...I only had one motor. This meant I needed a shaft to link one side of the bridge to the other, or I needed to put the motor in the span that moves up and down. An overhead shaft seemed like the best solution to me. The footings of the bridge are designed to guide the trestle into place. The chain for lifting the trestle is wrapped around a gear and held in place by a rivet. The trestle is a simple design and is strong enough for four lengths of track; this bridge is only three track lengths long. My definition of "strong enough" is if the span flexes it is not strong enough. Here are the gears that drive the chains. The motor is a 4.5V Technic motor. The bridge doesn't fly up, but then again draw bridges do not fly up anyway.