Does Concord Trailways go from Concord to TD Banknorth Garden?
Assuming you mean Concord, NH and the concert gets out before 9:30, you would take Concord Trailways to Boston South Station. From there, you would take the Red Line to Park Street and change for the Green Line (Lechmere or North Station), and get out at North Station. The Garden is upstairs. But note that the last Concord trailways bus leaves North Station at 10:00. Try Craigslist Rideshare to see if anyone is offering a ride.
