There's really 2 sides. By the parking areas are lots of short single and double track trails. Some nice parts but they all go up and down the same hill that is circled by park road. Many people so ride careful! The other side of 85 is also the park and is less travelled. Trails are slower due to leaf build up and lack of maintenance.
From the north: Take Route I-495 South towards Marlboro/Taunton; take exit 21A to Main Street Hopkinton. Travel down Main Street until the third traffic light located at the heart of downtown Hopkinton; turn left, traveling North on Route 85. After about two miles, the Hopkinton State Park entrance should be on the right.
http://www.singletracks.com/item.php?c=1&i=4444