Booking a train ticket out of Xian in advance
My family would like to take the Z20 soft sleeper out of Xian to Beijing later this month. I've read in Lonely Planet and elsewhere that it can be difficult to obtain seats if you only buy them a day or two in advance. I've also read that you can only buy China train tickets in the city from which you are leaving. Since we will only be in Xian a couple of days, that presents a problem.
I've emailed my fancy Western hotel in Xian and asked them if they could buy the tickets for me in advance and, to my surprise, they said they couldn't help me.
The only other service I know of is chinatripadvisor, and their commission appears to be pretty steep.
Are there any other alternatives? Are the fancy "Z" trains perhaps somehow different, so perhaps I can buy them elsewhere? For instance, I have contacts in Beijing who I could ask to buy the tickets for me, if that were allowed.