If you are in an organized private tour, the first thing you should do is to call your tour guides and let them to handle this issue.
If you are an independent traveler, you should report the loss of ticket at the ticket office of the railway station by presenting your ID certificate or your valid passport and pay for a new ticket for the same seat/berth on the same train. A service fee of 2 Yuan is charged.
Once boarding the train, you should tell the conductor that you lost your ticket and have bought a new one. The chief conductor will give you a passenger transport record before you reach your destination. With the record, the new ticket and your passport, you can have the new ticket refunded and get the money back within 24 hours after you arrive at your destination.