Why did Gilliam sacrifice his arm to save Edgar in Curtis backstory? Because it was a necessary sacrifice, to keep the supply of kids going, as living parts of the train.
Speaking of kids, following the children-replacing-broken-parts paradigm to its natural conclusion would mean that eventually the train would be running on an engine that would be entirely made of small children.
However, Curtis states that the cannibalism began after the first month aboard the train, and ended with the introduction of the protein blocks. This would be before the components of the train had begun to wear out. It would still gel, though, if Wilfred had intended from the start to replace them with child labor.
As horrific as the train's society is, Wilford and his minions aren't really wrong about anything. Everyone is only alive because of Wilford. If the engine stops running they will all freeze and die, at least until the ice age starts to end. Presumably they could and should have built a better and more just society, but on balance, Wilford's system apparently did prevent the extinction of the human race.
What happens to the children when they grow too big to work with the engine? They're not returned to their families, after all — and as Wilford points out, there's only so much room....
Perhaps they are reassigned to other support functions. Someone has to clean up after the raves.
There's also probably a pretty high mortality rate for that kind of work.
What do the cockroaches eat? Where does the waste from the train go? The tail-enders are probably eating their own recycled feces.
The fact that the train was designed with a cockroach-grinding, protein-extruding machine at all has some pretty dark implications regarding its origins.
Possibly it was originally intended to process animal feed for the chickens and fish.
what if that machine is responsible for the missing parts?
On that note, that the train's supplies included 50+ tomahawks and black ankle-length vinyl smocks is pretty disturbing.
Hey cool, there's still life around on planet Earth! It's a polar bear, the biggest terrestrial predator, a few dozen meters away from a teenage junkie and a five-years-old boy. Guess how that ends...
Everyone on the train was dead from the moment that the windows were shot out unless super armored glass is easier to make than an engine degreaser.
There's no explanation for why Gilliam worked with Wilford all along, especially given that he sacrificed his limbs for none of the comforts Wilford had. An one-off line implies Gilliam may have been a zealot about the plan to sacrifice the tail enders to keep balance.
It makes perfect sense if Gilliam believes the speech Wilford gave Curtis.
Or he may have reasoned that the likely alternative was Wilford simply ordering the entire population of tail-enders killed all at once, and recruiting future menial workers entirely from demoted front-end criminals. At least a revolution spread the mortality out a little more fairly around the train, and the rebels who died would do so believing they were accomplishing something.
When Wilford states that Curtis is the first person to have ever traveled the entire length of the train, he somehow forgets that the kids he has as replacement parts were taken from the tail end to the front as well.
Answered by more Fridge Logic. Given how he treats the children as spare parts and not people, he's entirely right in his thinking Curtis is the first person to have traversed the length of the train.
Or he's being nitpicky about how the children didn't walk the entire length: they were picked up and carried. It's a long train, and little kids who've spent their whole lives stuck inside a dozen-odd cramped boxcars aren't going to have much stamina.
The front-enders are way too indoctrinated in the Wilford cult, aren't they? Mason reacts to the schoolchildren's hymn like it's a childhood favorite, but it's only been 17 years. And no parents are objecting to the cult? Everyone converted religions in less than two decades? The movie seems a bit at war with itself; it wants its characters to remember the outside world and simultaneously act like they all grew up on the train.
Wilford himself suggests that everyone aboard is at least slightly mad. The front-enders are over-indoctrinated because they're trying to cope with both the end of the world and being intentionally kept in terror of both the tail-enders and the possible failure of the engine.
We only know that the tail-enders originally fought their way onto the train, not how the rest of the people on board were admitted. Could be that Wilford selectively recruited a bunch of fanatical devotees before the world froze, gave them all tickets, and then had any of them who might have developed second thoughts about his dogma tossed out into the snow.
As far as the frontenders were concerned, Wilford was basically the sole person responsible for saving them from a (probably) slow and agonising death being frozen with the rest of humanity. Not to mention that as soon as they got on board and the caste system was being established, Wilford probably made it clear that those who showed the most devotion to him would be likely to become one of his "inner circle" as it were, which they'd probably assume would mean more luxuries. It's actually hardly surprising that such a cult around him had been established within 17 years, particularly when there seemed to be a population which appeared to be mostly around 30-45 (for reference, Curtis is 34 and it's hinted quite a few times that he can't remember much of life before the train). People are inherently fairly adaptable; since the frontenders' experiences of life on the train seemed to be quite pleasant, why would they even bother trying to remember what was likely a very panicked and stress-filled life before that instead of just accepting that yes, Wilford chose to save them and they therefore owe him everything?
How did Timmy find a fur suit, complete with boots that fits him perfectly? Was there a stash of them in one of the first few cars?
There were furs in the luxury-cars where the drug party was going on. Presumably everyone who boarded the train originally had been bundled up in thick coats in order to survive the journey to the station, and discarded their outerwear once they got inside where it was warm. The furs weren't thrown away - Wilford's preaching aside, many people would've still been hoping the planet would thaw or the equatorial regions would prove habitable once the train got there - and were eventually placed in the party/orgy room as a bit of decadent decor. There'd be some child-sized coats among these, as the front-enders presumably brought their own kids with them; note that there are people in their 20s among the party crowd.