Enough about class politics; there's much more going on in Snowpiercer than just socioeconomic commentary. The film also reminds us that we're putting ourselves on the path to natural extinction.
Humanity has long seen itself as either the conquerors or shepherds of nature, one minute attempting to destroy the Earth and tasking itself with saving it another. Nothing could be more conceited. Planet Earth will exist long after humanity has come and gone, and there's no machine we can build to keep us around. Snowpiercer remind us of that.
"Inside the train, kids are taught that if you go outside, you're going to die," Joon-ho explained. "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."
Furthermore, it's actually Mother Nature which really puts humanity in its icy grave, not the homemade explosive of the film's climax. "It's the avalanche that kills everyone else," Joon-ho reminds us. "It's the revenge of nature, if you will."
And yes — in case you were wondering — the avalanche did, in fact, kill everyone left on board the Snowpiercer. "Yes, they're all dead," Joon-ho claims, "and that's a bit harsh. But it's a sci-fi film: If you can't say these things, or have these ideas in a sci-fi film, where can you?"