The 1st classies may have funded the train, but they're functionally useless now. This isn't a cruise or a plane ride--this is the survival of an entire species, with pretty limited resources that could be put to better use giving everyone a medium-ish standard of living instead of a handful of people a ridiculously lavish lifestyle and other people, invited or no, treated as more or less subhuman. Obviously the show has to require a certain suspension of disbelief, but there's no way that sort of system would hold up for long. Class and money no longer have any relevance in a post-apocalyptic world, where they need engineers and sanitation workers and have zero use for hedge fund managers or even sushi chefs.
"Wilfred" needed the first class folks to sign up, so "he" promised extravagant luxury, but once it became obvious that humanity was more or less extinct, other considerations presumably have to come into play. And while I don't have much sympathy for someone stowing away on a cruise liner, survival is the most basic instinct we have, so it's not exactly fair to expect the tailies to have just been like "oh, well, we didn't pay to live, so we don't have the right to live" before getting on the train. Especially since it's clear that this isn't a situation where there aren't enough resources to go around--there are, if they just bump the first class to like, a second class standard of living (I mean, saunas and sushi in the apocalypse? And the first class Marie Antoinette complaining about the nude Swedes in the sauna when the entire world is a dead frozen hellscape? Come on).