I think not having a "decently-sized terminal" is a much better approach. If I remember correctly (haven't taken a train across a non-Schengen border for a long time now, but https://travel.stackexchange.com/a/18563 seems to confirm), customs handling often happened on the train - the officers board the train at the last stop of country 1, do their passport checking etc. while the train was going to the first stop of country 2, and get off there.
Any move towards anything that would require a "terminal" as opposed to a platform will (on in the case of the Eurostar, was) just be the first step in making train travel as uncomfortable as air travel is today.
The Eurostar web site already mentions that you should be checking in 30 minutes in advance. I remember seeing the security checkpoint in London, shaking my head, and hoping this would not spread to other train lines.