City of New York Roads.
On a modern swing bridge, the center span is supported on a piston that is lifted up by hydraulics and then swung around with an arm (there are also locking devices at the ends of the swing span). The machinery that drives these early 20th century swing bridges are similar. As a bridge engineer, I rely on a mechanical engineer to design this machinery, although I suppose there may be some engineers who can design a movable bridge on their own. The operation of these bridges also relies on the Coast Guard who maintain the safety of waterways as well as with municipal authorities who periodically inspect the bridge (and the bridge operator). Thus, a movable bridge is a kind of hybrid which requires close cooperation between many different disciplines.
New York City's Bridges: 145th Street Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.