I took a train from Beijing to Xi'an in Jan 2003, but with my Sister and Dad. Not sure how we got out of Xi'an though.
I don't think you will have any problems on the train itself or at the station. Just watch your luggage like voyd said. In all the trains we took through China we had nice people sitting next to us, and were never bothered by anyone. So I would say go for it. You shouldn't run into any trouble and if you book a hostel before you leave, they might even pick you up from the station.
If it is a long trip you are up for (like 16 hours), get a basic, low class ticket that provides a bed.
How long will you have off, if you have to return to Xi'an?
"Travel is fatal to prejudice, bigotry, and narrow-mindedness."
"He who strays discovers new paths."