I want to pass through China by train from Vietnam to Mongolia. Does a free transit visa apply?

The information at the top of this page seems to imply it does although others have said this doesn't work by train. If it does, how do I insure a trouble-free passage?
"Free Transit
1. No visa is required for foreigners who hold tickets to the final destination and have booked seats on international airlines, ships, trains transiting directly through China, and will stay for less than 24 hours and do not leave the specified transit area."