Ooohhhh, 5-6 hours for a R/T to Tokyo is cutting it very close, by the time you go thru customs one way and immigration on the way back in. Narita is also under very heavy security right now. But if you want to chance it, it is best to know the train schedules in advance:

http://www.narita-airport.or.jp/airp...hedule_up.html

http://www.keisei.co.jp/keisei/tetud...-e/838u1-1.htm

My suggestion is that you take a train which stops at Narita town and enjoy the fun little shops, noodle bars, and perhaps take in the Narita Temple in the time you have available

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