The solid cement wall looking thing in the center is one of the counterweights that moves up the tower to let the deck down and up.
The bridge stays up to allow ship traffic to pass unimpeded, as there are way more boats than trains.
The gold emblem is the Army Corp of Engineers, who maintain the Canal and its bridges.
I checked the schedule for the Scenic Tour Train and discovered that the Bridge would be lowered around 11:30 in the morning, so I headed back down there to watch. There goes the engineer, into the Bridge to start the process.
While I waited, I checked out the informational plaque.
I had no idea that there was a bridge that preceded this one, when the Canal was much narrower.
Finally, the warning horns sounded to alert ship traffic that the Bridge would begin it's descent to track level. Here's a slideshow of the Bridge being lowered and then lifted. Incidentally, no trains ever showed up in either direction so I didn't get to shoot a train crossing the Canal.