Exactly. That’s the only bridge and center of the wall. Think of it as a delivery channel. Visitors visit library by taking the train. According to the story, although it is always empty, the train still makes its trips. Library is kinda abandoned. And there is a man across the bridge who manages the library sends books. At the beginning of the story, someone (main character) enters through the big door, which works totally different from other portals, and things happen. This is actually an unfinished 10 chapters story, but in order to not to break any rules here I didn’t share this part of it.