I've never once failed to get a ticket on the 3p train, and I typically buy at around noon on the day of travel. I have missed in several times though because the guy I usually ride with is habitually slow. Even this isn't the end of the world; we typically end up on a slower train departing 20 or 30 minutes later. If you're taking the subway, try to get out of Jing'an by 215p at the latest or Xintiandi by 205p (I'm being slightly conservative because both lines 2 and 10 offer Airport service on alternating trains... every 8 minutes or so). The subway actually is faster than the taxi IME, though you need to walk the entire length of the train station once you arrive... this takes close to 10 minutes, even if you rush, and I like to allow an extra 5 minutes in order to pick up some KFC/Subway for the trip. Train food is really bad, though the beer is not.