How did they film the train to train scenes

Coordinating two trains actually traveling side by side would be quite a feat. Also would be hard to contain the camera, crew, lighting, sound equipment, etc. all inside a compartment or even out in the hall.

Of course, building a set to represent the compartment seen close up is done commonly enough. But did they build a mockup of the other train and have it on tracks or rollers so it could move by the compartment from which the scene is viewed? Anybody know?

(movie AND train fan...)