There are several segments currently open - and more segments opening each year. Starting from the western most point in Northampton, the most famous and well-used segment is the Norwottuck Rail Trail.
Starting at the Connecticut River and heading east for 10 miles it terminates in the town of Belchertown, 1,300 feet east of the Amherst border. The paved trail is built, managed, and maintained by the Department of Conservation and Recreation (DCR)--the state's parks agency.
In the center of the state, lies the segment of the MCRT being advanced by Wachusett Greenways - (WG). WG should be the model for other grass roots, volunteer driven trail groups in the state. When they were came together as a formal organization in 1995, they decided to become a 501c3, non profit organization in order to get grants from various state agencies and charitable foundations.
In the ensuing 20 years or so, they have not only built and opened about 18 miles of the MCRT corridor, but they've been a leader in teaching other groups how to raise money, how to build friends and how to have fun along the way by hosting a series of fun family events each season. Here's a map of what they've built thus far.
There is an interesting section complete in the eastern part of the state in the city of Somerville called the Community Path - operated by the City of Somerville - is currently about 2 miles long. Here's a map of this segment.