Nope. Can't happen. The Central Mass tacks--
still connected to the Fitchburg Line--were refurbished for a few hundred feet back of the switch a couple weekends ago during one of the construction shutdowns into a new maintenance-of-way turnout. All the other such sidings west to Littleton were eaten by the double-track project and the spur right at Waltham Ctr. station was taken out for the downtown redevelopment. So they need this one as the only available staging area close to 128 for any routine track work. Turnout will hold a couple cars, and the grassy open space near the grade crossing is where they can stage materials (rail, ties, rock ballast, etc.) whenever they need to do work out there.
That'll create a permanent trail break between Linden and Beaver onto Route 60. Going further towards Waverley after the small detour around the block also has issues because of the private truck company driveway on the other side of the Beaver St. crossing. Any trail resumption from there to Waverley would have to directly abut the driveway and that company's building because of the telephone poles on the ROW. Company is not keen on that, and Waltham has low motivation to push the issue. So if Belmont successfully manages to get its rail-with-trail extension to Waverley Sq., look for some on-street solution + signage to bridge the gap. It's not that there'll never be a contiguous trail to Beaver St...it's that it's very low motivation for Waltham while they've got much bigger fish to fry.