Cutler Park is right across the street from my office. An 800 acre fresh water marsh along the Charles River, it is part of the Upper Charles River Reservation (MDC map), and contains a decent amount of single-track mountain bike trails. No big hills but enough small rollers (plus rocks and roots) to keep you workin.
I think I've pretty much mapped all the trails now. My favorite is on the west side of Route 128. Some good hills and the roots aren't as bad.
For those interested, you can also download the raw gpx data
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Otherwise, here's a static version: cutler park. Or alternatively here's a Google map version: cutler park
Generated via GPS Visualizer