Would the CR need to be dropped down at all for decking? I'd assume it would be decked at least to Beacon Street if not to Wilson Sq.
Also, if the GL was brought to Porter, it would need to duck under the CR right? Make Porter three-level. I assume then if it was ever continued to Watertown/Waltham, it would emerge somewhere after the Walden Street bridge.
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Aside from Porter, the parcels along the NEC and the Orange Line stations are screaming for TOD, air-rights or not.
Cambridge and Somerville have wasted no time studying this and holding community meetings:
http://www.somervillema.gov/sites/default/files/TRA-MBTA-AirRights06-13-2011.pdf
http://www.somervillema.gov/departments/ospcd/planning-and-zoning/porter-square,
http://www.utiledesign.com/projects/porter-square-air-rights/.
Note on the architectural firm's analysis they proposed setting the first floors of any buildings a few feet above street level to ensure proper clearances if the trackbed can't be dropped. They're intent on doin' this thing when they can get a state commitment for it. And of course GLX to Porter is an official long-term advocacy for STEP. So I wouldn't be surprised if they pitch a combined build as the quickest and most bang-for-buck way of getting this done.
As for how and where...the retaining wall disappears at the end of the car wash property and the Fitchburg Line returns back to ground level. So Wilson Sq. and the Elm intersection is the furthest possible extent of the decking. With the Beacon overpass the 'safe' fallback if Wilson's too big a reach. The Fitchburg trackbed was already undercut in this area once in the early 1980's. Freights used to have to divert at Alewife over the Freight Cutoff line that ran on what's now the Community Path because the Fitchburg bridges starting at Mass Ave. were way too low to clear the top of a freight train. When the Freight Cutoff went away during Red Line construction they raised bridges and did a lot of trackbed undercutting to get proper clearances the rest of the way inbound. You can see an earthen embankment appear on the retaining wall at the far edge of the Porter platform down to the Beacon St. bridge where the trackbed was dropped a good 4 feet:
http://goo.gl/maps/VFrwM. Another embankment appears approaching the Dane St. overpass. All of this was done one track at a time without any disruption to service, so there are no underground obstructions or utilities to relocate. Plus solid bedrock in this part of town and a Red Line that's more than 100 ft. below ground.
I think at most another 3-4 feet ought to do it for getting level clearances, and it wouldn't be everywhere because the prior undercutting left the canyon at variable depth with deeper cut clustering around the bridges. Porter station wouldn't need anything until you get past the old Somerville Ave. staircase. More from the edge of the platform (which one would assume would be torn up and redone as a full-high) to the Beacon overpass approach. Nothing in immediate vicinity of Beacon. A little more towards Wilson Sq. Not hard to do.
As for a GLX extension punching through the Prospect St. overpass's retaining wall...other than disruption to the Union Sq. substation the Fitchburg's full 4-track width here until Beacon St., so it's matter of shifting tracks over like they're doing for the existing Union extension. Maybe widening the Washington St. overpass, but Prospect and Dane are wide enough. On the Porter approach the easiest way to do it is slip into the air rights portal at Wilson Sq., then incline down on the Beacon approach before the ROW width starts getting cannibalized by the Porter platform. And then dip under the Fitchburg tracks into Porter ultra-shallow such that the GL tunnel roof is the literal Fitchburg trackbed with nothing more than ballast on top of the concrete roof. Then interface with the Porter lobby as stairs/ramp up to the CR level, stairs/ramp down to the GL level. Since it's shallow up and down would be about the same number of stairs. And then put tail tracks ending under the Mass Ave. overpass so the tunnel wall allows further extension.
Easiest to do as a combined build with the air rights because you can get the retaining wall, the trackbed undercutting, and the shallow tunnel + tunnel roof as new trackbed over with all in one fell swoop. Wouldn't be too expensive for all the combined economic benefits they'd get out of it.
Any which way they need to do something about that canyon dividing Somerville from Cambridge, and the Lesley campus from Somerville. I don't even care if there's no buildings on top of it. A linear park and bike path down to Beacon and Wilson Sq. connecting with the Alewife/Minuteman/Fresh Pond path network west of Mass Ave. along the Fitchburg ROW, a bus turnout on Somerville Ave. so the curbside pickup doesn't squeeze the traffic lanes, a Somerville Ave. egress from the parking lot of the Lesley/ex-Sears building and shops. Maybe a playground. Any of that would heal the scar and provide huge accessibility and economic boost to that corridor.