BJH, cynical 🔸
...amusingly enough you're not that far off of that last one.
There's two graphic novels in the series the film was based on.
In the second book, it takes place on another train whose passengers are convinced will eventually run into the first train, still circulating somehow in the opposite direction on the same world-spanning track.
And then they later find out that the earlier train is actually hidden /inside the second train/ and... yeah.