I think all the trails in PM Park are open. You could get to Eaton's from Faxon(there are a few trails in there) but I've only done it once--there's an opening behind some condos and from there its all parking-lot bingo to get all the way down--so far. That's all Quincy, which we haven't dealt with yet....
Otherwise, from Faxon you could ride the road up and over Penn's Hill (Franklin/Commercial St) to the tracks, go left and ride the RR right-of-way till you see a trail on the right. Just be aware that it is a live RR track.