Sci-fi thriller is equal parts visionary and cockamamie, but let’s appreciate the parts of its non-comformist vision, shall we? Snowpiercer is a provocative concept, a post-apocalyptic adventure where the last survivors ride a self-sustaining train that is on an endless loop. It employs the old story of underclass inhabitants exploited by the cruel upper class fascists. After 17 years, the weak and oppressed summon the strength to cast a revolt, with the goal of hijacking the front engine car operated by a mysterious overlord, known as Wilford the industrialist. Korean director Bong Joon-Ho (“The Host”) teamed up with writer Kelly Masterson (“Before the Devil Knows You’re Dead”) to adapt the French graphic novel “La Transperceneige.”

Chris Evans is the stout, vigorous hero of the revolt – although it’s not explained how he could be so buff living on gelatin protein bars as the sole food source for all the years past. Jamie Bell is the obvious plucky sidekick, resourceful and chatty, and lends motivational coaching unto the others. John Hurt, whose genius as an actor never recedes, is aptly cast again as the crippled wise sage who has ideas about how they are going to defeat all the guards and overthrow the train. Octavia Spencer, Oscar winner for “The Help,” creates a rueful mother who insists on joining the fight. Song Kang-ho and Ko Ah-sung (veterans of the director’s previous films) are the Korean tech heads that know some secrets on how to break through all the gates and turn out to be surprisingly useful in other ways, too.

Tons of bad guys in slaughterhouse uniforms fight off the revolters, which leads to grisly knifing and beating scenes. But it’s Tilda Swinton, as a secondary nemesis, who singles out as a totally weird villain, like a strict schoolmistress turned Nazi. She’s a standout sociopath, a death bureaucrat. The story does teeter on the fear at this point that once the good guys have taken hostages as leverage, it’s going to get repetitive – take a hostage, take the next state car. But the film shifts into bizarro comedy territory, and the grey-cold interiors that represented the underclass are replaced with something more imaginative, as we get answers as to how the train is able to be everlastingly self-sustaining.

Yet I’ll never understand why directors as good as Joon-Ho, per se, can build and construct an imaginative sci-fi concept of unusual circumstances and then tack on unnecessary elements that spoil the mood. The first-class residents get their own sleeper cabins and lavish amenities (it’s droll that twice a year they get to eat sushi), but what’s with all the mindless teen ravers just two cars from Wilford’s? You’d think Wilford would prefer being a room over to remaining intellectuals he could have a human conversation with! A few Capitalist elitists similar to the ones in “The Hunger Games” would have set the tone, but there are just too many damn extras playing useless and cockamamie teen deadheads worthlessly taking up space on the train.

Nevertheless, “Snowpiercer” has a lot of unique qualities going for it. When the director cuts away to the exterior shots of the world froze over, it is awesomeness indeed, with a matter-of-fact apocalypse happenstance that recalls “Quintet” (story awful, visually stunning) and “A.I.” (troubled, but still rather extraordinary filmmaking). And Evans turns out to have a little craziness in him which goes against the grain of prototypical righteous movie heroes. The violence is sometimes powerful. And also powerful is Evans’ quite brilliant monologue of “the beginning” when he first boarded the train years ago, right before he is about to come face to face with the destiny he has been waiting for.

I forgive the bad scenes of “Snowpiercer” just to get to the great ones. It has more original ideas than this week’s box office champ – Hollywood whore sellout “Transformers: Age of Extinction,” I promise you that. I only want to add I can’t rate this any higher because of those damn ravers.

126 Minutes. Rated R.

SCI-FI THRILLER / BLOODLUST / SATURDAY NIGHT WEIRDNESS

Film Cousins: “Quintet” (1979); “A.I.” (2001); “Time of the Wolf” (2003, France); “The Hunger Games” (2012).