Tube bridges are just bridges that are shaped like tubes. They can come in all different sizes and shapes (well, at least all different kinds of tube-shapes), and are used for all sorts of different things. Some tube bridges are just big enough for pedestrians to walk through and some are big enough to hold entire traintracks or even highways. Some tube bridges even run underwater, making a road for cars that's entirely submerged. Here's a picture that I found of the tube bridge at Ysterplaat Station. This bridge allows pedestrians to cross a busy highway:
(taken from http://www.asch.com/html/c_sea.html)
Like in this picture, the bottom inside surface of the tube is usually built up to make the bottom flat, since it's much easier for people and cars to move on a flat surface than a curved one. And if the bridge is totally sealed or extremely long, like many of the ones that run underwater, they have to have massive air circulation systems built into the walls, floor, or ceiling to keep the air inside the bridge fresh.
(published on 10/22/2007)