From I-495, take exit 31 to Rt 119 West and continue for about 7.0 miles to Groton center. Turn left onto Station Avenue before Groton Town Hall, drive to the end and park alongside the Rail Trail. Or take the Fitchburg commuter rail line to the Ayer station, then the trail: schedule is here: https://mbta.com/schedules/CR-Fitchburg/timetable