Provided you have a TTB permit, you can travel on the world's highest railway to Tibet. This means that you should book a tour that includes your train trip. Air-conditioned trains, complete with oxygen masks because of the altitude, arrive several times a week from Beijing, Shanghai, Guangzhou, and Xian to Lhasa. Every passenger has to sign a health waiver saying they're fit to travel. The rail link is not without its controversies—some argue that the train encourages Han migration, and that its construction has had a tremendous environmental impact.