The railway from Beijing via Xining to Lhasa in Tibet passes over the Tanggula Pass at 5,072 m (16,640 feet) above sea level (apparently the world's highest point on a railway) before dropping you at Lhasa at 3,490 m (11,975 feet) [1].

It takes two days at around 100 to 120 km/h over 675 bridges, though some recommend [2] that you spend a day or so in Xining on the way to acclimatise to the altitude.

Every train carries a doctor and oxygen to increase the concentration of oxygen in the carriages from 21% to 25% (but no pressurisation), plus a separate oxygen supply for each passenger.

I'm not a fan of China, but its rail network is something to behold.

[1] https://en.wikipedia.org/wiki/Qinghai–Tibet_railway

[2] https://www.yowangdu.com/tibet-travel/tibet-train.html