I apologise for alerting you unnecessary but what I want to say is that if no one answers this within a reasonable period, try www.lonelyplanet.com/thorn/branches and try the North East Asia branch (you may have tried it already for all I know), it's a message board and there'll be people there who are actually living in China at the moment. According to my guide there is a train from Beijing to Shanghai but you may need to do a changeover somewhere along the line to get to Hong Kong. Ticket prices tend to be more expensive depending on what type of seat you get, there are three classes, the third is a hard seat, literally apparently and very difficult to sleep in I believe, it's the cheapest seat going. The second is like a large dorm, you get bunks of 3 beds usually about 30 or so in a carriage, it's quite open and it's more expensive and you usually get families travelling together so it can be noisy. The first class, the most expensive, usually the province of party officials and anyone with money, you get four bunks in a closed compartment and better service from the staff. Third class fills up quickest followed by the second, you can usually book a day or so in advance and get what you want.