If I take a train in China do I need to show my passport? And which part are they checking? I know it seems a silly question, but how about for flying within China?

All bets are off during the Olympics, but up until now, no you don't have to show ID to buy a train ticket, but yes you do when buying an air ticket (and, of course, when checking-in).

Word has it that foreigners' IDs are frequently being checked at the moment, so if your train journey is to or from Beijing in particular you might well be checked.

The visa is not examined. Only the main picture page. (Hotels check the visa.)

Peter N-H

Small supplementary: If you're buying an international rail ticket, one to Hong Kong, or one to Tibet, you do need to show your passport or in the case of Tibet you'll need to show it to get a Tibet permit (youxing ka), and then to show that when you buy the ticket.

But all domestic flights require you show the passport, unless you're a resident of China with a work permit and have that documentation instead.

Peter N-H

Okay, so if I buy a train ticket they won't check my visa? I intend to go from BJ to Guangzhou.