[dredmorbius]

That's a problem already for passengers on China's Qinghai-Tibet railway line, whose highest station is above 5,000m (16,600 feet).

The railroad cars offer enriched oxygen and supplemental oxygen, though they're not pressurized. Edema is a real possibility.

https://en.wikipedia.org/wiki/Qinghai%E2%80%93Tibet_Railway

Or this travelog:

http://www.sfgate.com/travel/article/HIGH-TRAIN-TO-TIBET-Wor...

As the altimeter approaches 17,000 feet, a package of potato chips balloons outward until it ruptures a seam. Sunscreen and hand sanitizer erupt unbidden from bottles. In soft sleeper class, Chinese businessmen sprawl listlessly on their bunks, sucking oxygen from plastic hoses. The bathrooms smell of vomit.