The Warden HIMSELF (spoilers) ...
... climbs down in a rope ladder from a helicopter into a fast-moving train to catch (or more likely kill) the fugitives?
That does not sound even remotely plausible!
Indeed, there's nothing remotely plausible about this films at all. And yet it's great. Go figure.
Maybe it's because art is not about verisimilitude (otherwise painting would long have been supplanted by photography) but about reaching higher truths pertaining to the nature of existence, sometimes through a series of factual mistakes.
The death defying obsessive warden character is most likely a riff on Moby Dick's Captain Ahab. And of course there's white everywhere in Runaway Train: in a way, the great white whale in Runaway Train is the world itself.