F. Gilbert Hills Foxboro, MA
F. Gilbert Hills (or better known as Foxboro) has its reputation for being a challenge. The sheer amount of rocks can be staggering. However, I am told, over the past few years they have begun to open some new trails that won't make you feel like you ass has been in a prize fight. If you are looking for smoother singletrack try hitting the trails around Sunset Lake.
When I think of Foxboro, I think of rocks. Some of the trails here (I am looking at you Loop C) can really take a toll on your body. If you like rock gardens, head for those two loops. Now, I am told things are improving as new trails are built and old trails are revamped, I have a buddy that swears by Foxboro, it is basically about knowing where to go. Unfortunately, the park map doesn't even come close to showing all the trails available. It is best to hit the trails with someone who knows them well or hook up with one of the social rides.
If you are looking for tight technical trails and a challenge you will find it at F. Gilbert Hills. The black diamond trails can push even the most seasoned of riders. Head to the Sunset Lake side for more fast flowy, less technical trails. Whichever side you ride you will not be disappointed.
There is a small but perfectly placed parking lot at 111-115 Granite St
Foxborough, MA 02035 (map coordinates: 42.058388, -71.268477). This will allow you to ride both sides of the park without much backtracking.
You can also park at the park headquarters (45 Mill Street Foxborough, MA 02035) though be aware this parking lot is usually full on the weekends.