Train tickets all sold out? Can't keep constantly checking back to see whether returned tickets have opened up any places?
Relax. China Rail has finally instituted a waiting list system, though it's only available for bookings on 12306.cn and its app.
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Of course since booking on 12306 and the app requires a reasonable level of Chinese and a Chinese bank card, this might be just one more way of putting foreigners, many at least, in a disadvantageous position.
On another point, booking sleeper trains has always been a toss of the dice if you're looking for a particular berth, middle not upper, say. The system automatically assigns the berth, and while you can cancel and try again if you're not happy with the assignment, you can only make and cancel three bookings each day before you're locked out till the next day.
But I noticed that now, on the app at least, when you select a train but before you make the booking, you are shown the exact fare of the berth you'll be assigned. So with a bit of research you'll know from the fare whether it's upper, middle or lower, and if you're not happy you can just start over without running afoul of the three-per-day rule.