Two things: (1) We want to emphasize not only how dependent Christopher is on his stupendous brain power, but how keenly aware of it he is at all times. (2) We have to say something about the open Swiss Army Knife. Does it represent just how intensely frightened he must be feeling to be prepared to stab (or at least threaten to stab) someone? Or does it show how incapable he is of empathy (that is, that hurting someone else would be a bad thing)?

Once again, Christopher confines himself into a small space, and that makes him then feel safe. At the same time, though, it affords him the freedom to peek out through the curtain, and to watch how other people use the subway ticket machine. So, it sounds as though he benefits from finding freedom within confinement.