Apparently you don't get out of your home country much. Train tracks have long crossed multiple international borders. Custom checkpoints are setup at airports and seaports all over the world. It's entirely feasible to setup the same infrastructure for the ETT. To your other point, existing transportation vessels like airplanes and cruise ships require constant maintenance as well but they operate just fine.

To Carpe: I've traveled to all seven continents. Train tracks are one thing. A sealed tube is an entirely different proposition, particularly when the cars inside are traveling at hundreds of miles per hour. The purpose of this tube travel is uninterrupted speed. If you are stopped at every border for identification and papers checks, it defeats the purpose. Here's a thought. Train tracks are made of steel and wood. That amount of materials are miniscule compared to that which will be required for this tube project. As for maintenance. A single ship or airplane can be maintained without problems. Thousands of miles of tubes which will have to be inspected regularly will prove to be very expensive.