Quote Originally Posted by iheartthed View Post
The customs agents could perform their checks on the Detroit side before passengers boarded the trains.
The US does this with Bermuda and the Bahamas.

So that flights from these close-in airports can go into airports without international terminals or with inadequate space in their international terminals, the US stations immigration and customs personnel in these countries to pre-screen the passengers as they board the flights.

Switzerland does this on the TGV from Paris to Lausanne. The Swiss personnel board the train at the last stop in France and conduct all their business while the train goes to Lausanne. When you get to Switzerland, you just get off the train. I believe that this is done by other European countries as well.