Beijing to Lhasa, the Tibet Railway.

The 1,338m Fenghuoshan tunnel is the highest rail tunnel in the world, at 4,905m above sea level. The 3,345m Yangbajing tunnel is the longest tunnel.
Around 1000km, or over 80% of the railway, is at an altitude of more than 4,000m. There are 675 bridges, totalling 159.88 km, and over half the length of the railway is laid on permafrost. Permafrost turns to squishy mush if it melts so the engineers use many techniques including liquid nitrogen pipes and metal sun reflectors to keep the soil under the tracks cool.
Lhasa to Beijing - 47 hours and 28 minutes, 4,064km.
Lhasa to Shanghai - 51 hours and 39 minutes, 4,373 km.
Lhasa to Chengdu - 48 hours and 50 minutes, 3,360km

Cheers
Paul