There's just something inherently cinematic about trains. From the Lumière Brothers' film of a one arriving, to Edwin S. Porter's Great Train Robbery, to the Cinerama splendors of How the West was Won and beyond, trains and moving pictures have been inextricably linked. It makes sense. If you're making a movie, you want things to move. Also, the continuous forward motion mirrors the driving narrative of a well-told story. Yet often forgotten is the fact that trains make stops.
Sure this train is headed to Busan, but there are plenty of stops to make along the way to give the viewer some variety. And not just variety of location and action either. This "horror" film makes protracted stops at the "family drama" station, the "teen romance" station and the "slapstick comedy" station on the way to its conclusion. At one point it even becomes an extremely current allegory for the plight of refugees. This movie gets that the journey is just as important as the destination.