Somewhere far out of sight, north of here, was the "black hole", a vast stretch of Cultural Revolution era prison camps; some still in use apparently. I'd read accounts from Tibetans that had spent time in these camps. Horrifying stuff. The section of track that the train had passed over in the night was built by these prisoners. And now it was this very railway was flooding Tibet with migrants from mainland China, to the point that Tibetans were now a minority in Tibet.