For every anti-rail NIMBY there's an anti-trail NIMBY. And the worst are the ones who oppose both and buy, build, or squat over the routes. Those are the real ones to fight.
For every person conspiring to block rail service by building a trail, there are dozens upon dozens of trail advocates who want trails to ride on, nothing more. Trail advocates who want trails on key routes for future expansion nearly all want them for the same reasons people on the forum see them as key routes for expansion: they go where the people are, in a reasonably straight way. Especially in suburban areas (like all of eastern Mass), long, connected trails are the difference between a skinny park for light recreational riding and a real route for light recreational riding AND serious cyclists, people touring, commuters, etc. Those last three are pretty small right now, but they are real and they have a lot of growth potential. Another phrase for them is "alternative transportation"; they are much closer to commuter rail advocates than to highway advocates. Hating trail builders and demonizing them and assuming they're all plotting to kill rail service can end up preventing real cooperation that should take place. Sure, watch out for and expose actual conspiracies and bad design, but then mostly work with other people who are likely to be on your side when it comes to building better sidewalks, putting bike racks in commuter rail cars and at stations and on buses, using gas taxes for something besides more highway lanes, etc.
Constructive ideas for Newburyport project:
Write to T and town of Newburyport to lobby for good bike racks at Newburyport Station, possibly with a roof and/or with bike lockers. (Ogden, UT, has coin-operated bike lockers at its new Frontrunner commuter rail station. The Dutch have good bike-train/transit connections, and we could too, especially when there are condos a one-mile ride away from the station on a safe, direct route.
Pester local bike rental places to rent bikes to tourists at station. Pester town of Newburyport, Chamber of Commerce, etc., to put up bike racks and designate bike routes through town, to make Newburyport a train + bike destination. Can you bike out to the shore from the station? Pester MBTA to market BON-Newburyport train+bike outings. Anything that keeps up ridership is good.
Constructive ideas for other projects:
Get in touch with those groups, especially if you also like to ride a bike. Talk about how to build a trail that will still be safe and intact if a line of railroad is ever rebuilt. Point out the possibility of connecting trails to active stations so that cyclists can come by train or passengers can go by bike and lock up. (I've written to the Rails-to-Trails conservancy about this; they can be infuriating in their magazine, which will do something like tell about the great places you can bike from Burlington, VT, without menthing that you can go their by train with your bike pretty cheaply. Talk to selectmen, city councillors, etc., about the potential for integrated intermodal transportation (trails and trains working together).