Separate trains will also help speed things up. With one train, they have to wait until everyone has cleared customs and immigration before continuing on, but if they are transferring to a different train, they can schedule the departure of the second train for enough time for most people to clear (similar to transferring at an airport). If someone is getting grilled, the second train can leave on schedule, and that person can catch the next train. Wort case and the person doesn't clear immigration, they can take a train back to where they came from (harder, though not impossible, to do if the train is continuing on in the same direction).