Interesting to see this thread go on for years...

Here's my take: Casey is an addict, he's most unhappy with his life, in need of a drink, tense and easily upset. He's having a hard time making his way through life, feeling alienated to all the people around him, so he needs a lot of energy to concentrate on his way ('minds the arrows') while trying to block out any disturbances (the fatal echoes...) and especially the other people who 'rattle his chains', i.e. upset him, probably by their mere existence. This is not exactly about riding the underground, it's more like Casey's whole life feels like he's in a tunnel, with no way out. The 'turnstiles' are a probably a symbol for crossing a line, for the feeling of being trapped with no way back (or out).
He's obviously unhappy with his family life, but the encounter with an old lover does not do any good either. This encounter is most likely a fantasy, but he's so depressed, he can't even come up with a pleasant fantasy anymore. Casey wishes nothing more than to end it all, finally taking the 'last ride', as in committing suicide. Let's keep in mind he's a loser and a drunk, so he's most likely unable to put an end to things.

Does this make any sense to you?