I don't think they need to show how society functions normally before they burn it to the ground because they went to ridiculous lengths to hold the economic social order on the train in stasis to be similar to what the viewer is familiar with despite seven years passing. In an apocalypse, its ridiculous that it hasn't already broken down.
I even hand wave that the reason this show is set on a train is because Wilford or Melanie realized that needing a environment to survive in that required specialized skills of the upper classes as a deterrent to an uprising that would keep the status quo where they remained in power longer.
Third would be more than upset. They would become the tail. If they cut loose the tail, third simultaneously becomes the tail and has seen that those in power are willing to cut the tail loose.
I sincerely doubt that the tail has been given any meaningful opportunities to contribute to the train. I think those who got out of the tail had something to offer (a skill or a bribe) and no one to leave behind or no one they were unwilling to leave behind. Taking some of the kids from the tail for internships is likely more a carrot to give a glimmer of hope that things might someday change to keep the rebellions down than anything else.
That parents are willing to turn their kids over for the hope that they will have a better life is a pretty good indicator that the tail isn't filled with a bunch of people that only want handouts. Another is that they asked for equal work and equal rations. That is a much better starting point than I want equal rations while not doing anything. If they aren't capable of equal work, then teach them.
I also don't think the assumption can be made that they can't contribute. These are not the dregs of the Earth that got on the train. This is a cross section of people that were highly motivated to survive and managed to do so. We know at a minimum there was a homicide detective among them, I don't think its unreasonable to think there are others with skills.
I think its more likely that there aren't enough resources to give into the tail's demands because the consumption of the rich is less in total than what they would need to make things more equal. Or they know that changing the social order will have such push back as being "unfair" from the paying passengers that survival will be compromised in the upheaval that would come with a change.
But when does it end? When do the money people get the return on their investment and the work of running the train becomes the larger investment. Do they get to pass down their passenger class to the next generation, the one after that?
The third class is effectively indentured servants for life. The tail is being sterilized so they won't have any future generations that might be able have a better life.
Maybe there is no plan for a future that is more equitable in terms of monetary vs human capital because humanity won't last that long, but with seven years having passed with the idle rich doing nothing and being able to get away with serial murder I can see how third and the tail would not be satisfied with their lot and want more.
My personal sense of morality on this one is everyone "deserves" an opportunity to work / learn and better their situation. I think the train has rigid social and economic structures that prevents this but gives the illusion of hope that voices will be heard or circumstances might change.
As I said at the beginning of this post. They worked very hard to start this show being very close to how the world works despite seven years passing since the freeze. If I look at this from the perspective of each class, I can see that they all have some valid points. But the tail is bearing the brunt of "life isn't fair" and I think seven years in a dark over crowded box car is long enough that I'd be thinking that its time to provide opportunities so they could better their situation.