There is no direct train from Hongkong to Changchun, you will have to change in Beijing or Shanghai. Hongkong to Beijing demandes 24 hours and to Shanghai 20 hours. I suggest that you go via Beijing because it's much nearer to your destination and this journey takes 6 hours with the bullet train (310CNY for the first class and 372 secod class), about 9 hours with the express train (150 for the hard seat, 206 soft seat, 230 hard sleeper and 360 soft sleeper) and 14 hours with the fast train (almost the same price as the express train). Are you coming soon? Because the travel peak of the year due to the spring festival will come in a few days (probabley from the 11,january, 2009 and lasting a month)and during that periode, the train tickets are hard to get everywhere in China!

PS: the ticket from Hongkong to Beijing costs about 600 CNY for the hard sleeper and 934 for the soft one; from Hongkong to Shanghai cost about 520 CNY for a hard sleeper and 825 for the soft berth. The tickets with a litte discount are available from time to time.