King Dando explained it perfectly and I can't do anything but agree with him. I used to think that good behaviour was all that was needed, but the tickets probably cost as well.
I just have to wonder. Meche had no money (as she never made enough to even pay taxes) and yet Copal says that she had a Double N ticket and shouldn't have had to walk because of that. Still, I think Dando's explanation is the best one. It's also supported by that Nick (who we can assume had lots of money) went down with the train.
Damn. Every time I think I'm done with this post, something else come to my mind. Anyway, the tickets Nick and others on the train had bought were fake, right? One could speculate that this is the reason the train jumped off the tracks and that they might have made it if they had had real tickets that they had bought from their legal owners. I personally think that they still wouldn't have got to the Ninth Underworld, but it can be argued (based on some lines in the game) that you need money to get the ticket, but the opportunity is only offered for those who are worthy. Even though it's forbidden to sell your ticket (as the Gate Keeper says), I'm wondering what would happen if someone unworthy tried to take the train with a real ticket.
But it probably wouldn't work because the tickets recognise their owner, so there must be some system to find out who is worthy and who is not... What do you think happens if everyone else but one person on the train has a real ticket they've earned, and that one person has cheated? They can't send all those people to Hell (or whatever the punishment is) because of that one soul, so do they just stop the train and kick that person out?
Now I'll stop.