The train has to keep moving because Wilford wants it to keep moving, or believes it must. The analogies and technical details aside, this is pretty much the point of most of the movie, in-plot.
Wilford is obsessed with maintaining a given order. He may be benefiting from this order; and he may be correct that there is no survival without it. (Though there are plenty of hints that may not be. Nam thinks he’s not, but Nam’s also a junkie. Even if he were to say he saw signs of life or warming outside, nobody would believe him. BJH is not in the business of giving his protagonists clear moral justifications, just like his antagonist isn’t obviously callous or mad.)
All this is secondary to Wilford being a fanatic. But even if this blinds him to how good the first class passengers have it, and the suffering inflicted on the people in the tail, and even were he delusional about the train falling apart or about mankind’s chances outside the train, the train is of his design. What it does and how it does it is under his control. This extends to almost everybody living on the train - the idea of life outside of it is inconceivable to anybody but Nam.
If there was a fairer/safer/otherwise better way to do anything the train does, nobody inside would know about it, and even if they did, they wouldn’t have the means to accomplish it.
Basically, world-building in Snowpiercer isn’t really about explaining how the train works, it’s about depicting a society ruled by an absolute despot like Wilford. The train is a major instrument through which he wields power; when the movie seems to tell you things about the train, it’s mostly telling you something about Wilford. That other arrangements (like a parked train) could exist within the same world is not something it explores, because it’s about living inside a physical and ideological prison that is, in-plot, constructed around the protagonists.
The audience is meant to make its conclusions without the convenience of knowing anything those inside don’t. This is why the polar bear shows up in the very end - beyond being a cliché “they all die” ending, it’s also the first reliable piece of evidence that life is in fact possible outside the train, concluding an arc of instilling doubt in this imprisonment being justified.