At South Station, go into the subway entrance and down one level. There you can board a Silver Line bus. The fare is $2.75 per adult (kids are half price, I believe) and tickets are available from vending machines (cash or credit cards okay). All three SL routes will take you to your destination. The bus will go through a tunnel and the second stop is World Trade Center. More info at www.mbta.com
Trains only go as far as South Station. You would need alternate transport after that.
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