I read about 20 from the currently 350 reviews here and as far as I've seen, nobody seems to mention 2 key scenes which seemed very important to me, so I'd like to add my two cents here, a different attempt of interpretation.
First key scene: the teacher scene. We witness how completely the children absorbed the doctrine of Wilford. The train manager became a benevolent deity, the system is never questioned, and the train must go on forever. The teacher is a sycophant, very much like Mason. What future do the children have if they are not allowed to think critical?
Second key scene: Wilford's monologue when he explains how sacrifices have to be made (only by others, i.e. the lower class). If the system can only continue by murder, then mankind survives by inhuman means, although Wilford sees himself not as the leader who created the train and the system, but as just another cogwheel in the machine. Therefore he expects that a younger man might replace him when he is too old, without any change to the system. He cannot imagine any alternative, but others can.
In conclusion, what matters is that the train must be stopped somehow to get out of this treadmill. Other reviewers have asked: is it possible to survive in the cold outside, wouldn't it be safer to keep going with the train. I think they fell right into the trap here that the movie makers prepared. Because just like the train is going in circles, returning every year to the same places, we are living our lives in circles, safe but in a society that loses its social qualities, as you can see quite often in the treatment of children or elderly people, for example. The revolution, as the film makers describe it, wants to break the circle and start a new life that may be less safe, but challenging and more human. I think people who ask: "wouldn't they freeze to death outside?" take the allegory too literal again. The frozen world outside represents the unknown, something you take careful glimpses at from the window, but whether it's 5 degrees below zero or 25 is hardly the point. The train itself was a better allegory than a ship or an airplane, because these do not strictly stay on the same tracks when they sail or fly around the planet. The polar bear at the end shows that life outside is possible, but one must try and face the unknown instead of continuing the same old evil.
I liked 'Snowpiercer' a lot as it gives much food for thought, but I wouldn't call it a flawless masterpiece (voted 8 of 10). The killer shooting through the window, the decadent upper class passengers in swimming pools or the traitor among the lower class passengers are too much of movie clichés. The question of how food is produced was answered much more terrifyingly in 'Soylent Green' decades before, in 'Snowpiercer' it was just good enough for a quick shock effect. But don't these little niggles stop you from watching this very interesting movie.