It takes ≈24 h to travel the ≈3000-km-long Qinghai-Tibet railroad of which 85% is situated above 4000 m with a pass at 5072 m. Each year about 2 million passengers are rapidly exposed to high altitude traveling on this train. The aim of this study was to quantify the occurrence of altitude illness on the train. Three subject groups were surveyed: 160 Han lowlanders, 62 Han immigrants living at 2200 to 2500 m, and 25 Tibetans living at 3700 to 4200 m. Passengers reached 4768 m from 2808 m in less than 1.5 h, after which 78% of the passengers reported symptoms, 24% reaching the Lake Louise criterion score for AMS. AMS incidence was 31% in nonacclimatized Han compared to 16% in Han altitude residents and 0% in Tibetans. Women and older subjects had a slightly greater risk for AMS. Most cases of AMS were mild and self-limiting, resolving within days upon arrival in Lhasa. Some cases of more severe AMS necessitated medical attention. To curb the health risk of rapid travel to altitude by train, prospective travelers should be better informed, medical train personnel should be well trained, and staged travel with 1 to 2 days at intermediate altitudes should be suggested to nonacclimatized subjects.