How are the passengers on the subway characterized?
The only place where it appears that people on the subway are described is in the midst of a conversation between Montag and Clarisse. They are talking about the people that they see and Clarisse says that she listens in on their conversations sometimes on the subway or in the cafes or other places.
What she says is that they always talk about the same things, basically nothing. Montag is shocked at this, suggests that it is impossible but Clarisse goes on to describe their conversations. She says that they just name a bunch of stuff, objects and toys and swimming pools and whatnot and then that the stuff is pretty cool. And that is the entirety of their conversation, they don't talk about anything else.
It would seem that she is trying to point out some of the same things that Montag feels, namely that the people are unable to even understand or talk about the emptiness they feel because they've been conditioned to fill it with things.