He trembles, but must investigate as high as he can climb. (16)
Aww, looks like the Man-Moth forgot his jacket. Wait, no, he must be really scared of that climb if he's trembling. No matter—up he goes, because he must.
[...] this time he will manage to push his small head through that round clean opening and be forced through, as from a tube, in black scrolls on the light. (19-21)
Each time he comes out, he firmly believes that this time he will make it. He fantasizes about what it will be like once he makes it, and it's going to be glorious.
The Man-Moth always seats himself facing the wrong way (29)
Wrong schmong. This is how the Moth-Man rolls, and he doesn't care about what anyone says. Maybe he's superstitious. It's like wearing the same underwear to all the football games in a season when your team is on a winning streak. Hygiene aside, there's a reason why he does things this way, and he will continue to do them until he accomplishes his goals.
Just as the ties recur beneath his train, these underlie his rushing brain. (35-36)
There's a great rhythm here created by Bishop's words. In fact, most of the line is written in iambs, which mimics the sound of a train perfectly. The rhythm allows us just the tiniest glimpse of what's going on in the Man-Moth's head and what drives him to try and try again.
[...] He regards it as a disease he has inherited the susceptibility to. He has to keep his hands in his pockets, as others must wear mufflers. (38-40)
The temptation to give up lives in all of us, because sometimes it's just plain easier to believe we can't do it and walk away. The Man-Moth acknowledges this, but he considers it a disease, a thing to be prevented at all costs. If it means that he has to keep his hands in his pockets when the temptation is near, so be it. On the up side, he'll save a little cash since he won't have to buy mittens.