By Mary Thornton
To begin with, Snowpiercer takes place 18 year after the entire world is consumed by sub-zero temperatures and the remaining chunk of survivors are forced to mount a constantly-moving train that completes a round-the-world, nonstop journey every 365 days. At the front of the train are the first class passengers: the former kings and queens of industry in the world-that-was who were able to buy their way onto the train.
At the rear of the train is everyone else.
Oh yes, final spoiler warning.
I found it fascinating that the chosen ending for what could have been a fairly straightforward, if innovative, science fiction action film implied not only the destruction of almost the entire human race after alluding to the cannibalistic nature of their survival in an enclosed space (and yes, that does end up being literal), but also the isolating effect that freedom from that system imposes on those who are the weakest. A child and a teenager who until recently was imprisoned for drug addiction aren't the ones you would expect to survive in the catastrophic conditions laid out for them, but director Bong Joon-ho believes the ending to be optimistic, saying "...extinction is a repeated word throughout the film. But outside the train, life is actually returning. It's nature that's eternal, and not the train or the engine, as you see with the polar bear at the end."
As far as apocalyptic predictions go, the relative permanence of machinery, class-ism, and steel is rife with possibilities. While I felt the futuristic elements of the movie were used to great effect both in making this world and justifying the action of the characters, I've noticed many reviewers are loathe to use the term "science fiction" at all when it comes to this and other similarly "cerebral" films. Denying the use of science fiction genre tropes does nothing but take away important vocabulary with talking about why movies like this work.