The federal transportation act sets aside a certain fraction of the total transportation funds to be used only for transportation improvements that do not involve motorized transportation. A large fraction of the construction costs for rail trails has been federally funded. The Federal funding is specifically designed to encourage alternate transportation--that is, the reduction of automobile traffic.
Massachusetts has recently completed the 2007 State Bicycle Transportation Plan ( see http://www.massbikeplan.org/) describing the state's commitment to bicycle infrastructure. As Secretary Cohen states:
"[The Plan] represents the most complete inventory ever compiled of existing on-road and off-road facilities and projects in the pipeline. Second, it develops a prioritized plan of on- and off-road bicycling improvements in order to implement a statewide bicycling network bound by a single identity. The network will serve to improve multi-modal transportation generally and bicycle transportation specifically."
This plan specifically identifies the east-west corridor in Sudbury (the Wayside Rail Trail) as a high priority corridor. It identifies the north-south line (the Bruce Freeman Rail Trail) south of Chelmsford as a secondary right of way. It should be noted, however, that this is only a plan. There is considerable pressure from towns along the BFRT to proceed with its development. At this time (September, 2008) Acton, Westford, Carlisle and Concord have all submitted to MassHighway 25% designs for their sections of the trail. All of these designs were begun before the 2007 Mass Bike Plan was made public.