Maybe I missunderstand what you meant above, but in PEK T3 you're perfectly allowed to stay airside. There is a transit passport/boarding pass check, followed by a transit security check one level lower, which gets you directly airside (you'll exit left hand of the regular security check) - no need to take the train, no need to walk much, just exit plane, have your passport and boarding passes checked, go one level down, security check, and you're airside. Definitely NOT "entering" China in the process.