I don't think that this strategy works now.
The best strategy now is to buy with a departure from a previous station where there is availability (or a later station). This is a very common trick for the Pingyao-Xi'An overnight trains.
I once took a train in Jiayuguan but waited too much before buying my tickets. So I had to buy a ticket from Lanzhou. The departure in Lanzhou was at 5pm but I got on the train at 3am in Jiayuguan. Our soft sleeper compartment was locked and waiting for us.
The only hesitation was due to the fact that the date on our tickets was the day before when the train actually departed Lanzhou but that was cleared up quickly as the conductor figured out that it was the same train, just that it left the station 9 hours earlier.