Yep, it's pretty simple.
When you get out of the airport terminal, there should be a silverline stop right as you step out the door. The Silver Line is pretty much a bus even though it's a part of the subway station. I believe you can just go on board and pay in cash if you don't have a charlie ticket/card. Just keep a few dollar bills on hand for this ($2/ride)
Here's info on the Silver Line:
http://www.mbta.com/schedules_and_maps/subway/lines/?route=SILVER
Once you're on the silver line, just stay on until you get to south station. Just get off and exit the "T" (subway) area by going through those little gates. Then go upstairs to the main area and you can get your tickets for the commuter rail. You want to go outbound (the only way you can go really) on the Framingham/Worcester Line. The Grafton Station is the second to the last stop.
Here's the schedule for the commuter rail:
http://www.mbta.com/schedules_and_maps/rail/lines/?route=WORCSTER
Commuter Rail's a pain in the donkey... but it's not too bad if you compare it to a MASSIVE cab fare you would have to pay to get to Tufts.
The T can be a bit confusing at times because it's sometimes hard to tell at what point you're going inbound/outbound if you don't know your way around, and the green line can be kind of tricky too. But as long as all you need to do is get to Grafton from the airport, it's really not that bad.
As for luggage, Silver Line is pretty much meant for transport to/from the airport so it's not bad at all. And as long as you're not going during rush hour, the commuter rail is very roomy.
Hope that helps.
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