We had our guest house purchase ours in order to get what we wanted.

Yes, we have used chinatripadvisor dot com site for our overnight train tickets from Beijing to X'ian in 2007. The tickets were delivered on the day we specified to our hotel in Beijing. Sure we could have waited till we were in Beijing and gotten them a little cheaper but we were in for peace of mind and not caring about saving $5.00. We also used cta for a private guide service in X'ian and were very pleased with their service. You can see that tr here:

http://www.fodors.com/community/asia/mr-mrs-ht-go-to-china-and-japan-2007.cfm#last-comment

Aloha!

Rules about buying trains tickets have changed a lot since 2007 and so have agent fees.
Passports are required to obtain train tickets. Tickets go on sale online 12 days prior to departure and in train station and ticket booths 10 days prior.
Ticket availability is hard to predict. It depends on the route, time of the year or some random factor like a group that happens to buy all the tickets. For some routes that have many trains like Beijing to Shanghai, it is not a problem. For other routes, it is always better to have a plan B.
I have seen many occurrences recently where tickets are sold out in the first three hours.
Asking your hotel is a good and cheap option but very few hotels will do now under the new rules. Personally, I would not rely on the fact that they will remember to go online at 9am when tickets go on sale. Ticket agents are more reliable but they all specify in their websites that there is no guarantee that they will be able to get the tickets. Hence the need fo a plan B.
The website where you can buy tickets does not work for tourists as not only is it in Chinese but also you must pay with a credit or debit card issued by a Chinese bank.
I have not used any agents myself as I buy my own tickets online but from what I have seen there are two types. The first group such as chinatripadvisor.com deliver to your hotel. However they need a copy of your passport and require payment in advance and are rather expensive. You can also get some to buy the tickets and email you something like an eticket (confirmation) that you exchange at the station while showing your passport for the paper ticket. In this case there is no ticket delivery and they only require payment after they have secured the tickets. They also charge less. One such agent would be china.diy.travel@gmail.com
One new thing also is that you do not need to be in the departing city to get your tickets any more. If you are picking up tickets in the city other than your departure city there is small 5RMB surcharge.

Peter - thank you for sharing your report. I somehow missed it earlier. Looking forward to meeting you and Linda in Shanghai.