The Cambridge Tunnel

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Re: The Cambridge Tunnel

Postby ariof » Thu Jul 26, 2012 11:32 am

ThinkBoston wrote:
MBTA3247 wrote:The Cambridge Tunnel is only just below the surface. Given the amount of disruption involved, it would likely be easier to send a relocated Grand Junction branch below the Red Line, rather than digging up both routes. Of course, that would require some relatively steep grades, especially on the north end of the tunnel approaching Cambridge St.


I imagined such (the depth), but thanks for confirming that. As you identified, it would require a rather steep grade for the Grand Junction line to pass under the Cambridge Tunnel, this is why I suggested the reverse action of dropping the Red Line which can have considerably greater rates of grade change. And the alteration would need be for only a short stretch of the RL, less than 1500'. It would likely be better to construct the underpass north or south of the current tunnel, if room allows. Main Street along this stretch, from Ames to Portland St., is roughly 80-90' of public right of way, considerably more than what the lines in central Boston channeled.

Being that my proposal for using the Grand Junction is to shift the Framingham Commuter Line, and with it a diversion of the Amtrak LSL, to North Station, grade changes need to be very moderate. The line is already lower upon crossing the Charles River, it would be relatively simple to drop the rest of the run through Cambridge to that level to pass beneath the few roadways it intersects and then rise it its final run along the MBTA rail corridor.


You'll never really be able to relocated the Red Line without major construction costs (think billions). You can't run buses in that section—Central (the last mainly residential station) to Kendall (mostly business)—which is the busiest on the line (probably 100k passengers per day, or a crush-capacity train every 4 minutes at rush hour). The whole of the line is cut-and-cover just below street level, and probably extends little more than 20 feet below grade. To tunnel under it, you'd need to tunnel down about 40 feet, maybe a bit more. At a 2% grade, that's about half a mile (give or take).

There's plenty of room for a portal from the west side, as the Mass Ave crossing is about 0.7 miles from the river. Even if MIT wanted to bury Mass Ave, you could get down 40 feet by that crossing and run level to Main Street (a good spot for a station). Going east/north, you could gain elevation and pass under Cambridge Street just below grade, and reach grade just around the McGrath (if Medford Street were closed), although you might want to tunnel below the McGrath and come to grade in the area of the Engine Terminal. (The current Grand Junction line crosses over the Fitchburg Line to reduce the radius of that curve.)

As far as tunneling, it would probably be a pretty simple slurry wall cut-and-cover job as long as you didn't need to maintain rail service. The track was built in 1846 when most of the area was marsh, so there are likely few if any conduits below it except under major roadways and between MIT buildings. (A complicating issue: the entire area is fill and 10 feet above sea level, so you'd be tunneling below the water table, but nearly every tunnel in Boston is built below the water table, so it's par for the course.) Rail connectivity would have to be maintained between the two halves of the MBTA system, via the line to Worcester, the P&W to Ayer and the Fitchburg Line. Long, but not unsustainable for a couple of years, especially with capacity improvements on these lines. It would make sense to electrify this line of course, and to use it in the future as part of a South Station-Back Bay-Kenmore-Kendall-North Station (and perhaps one day back to South Station) loop. This would require a branch off the bridge over the Charles and under the Mass Pike to tie in below the existing Worcester Line which would be difficult but not unfeasible.

Ridership on a Grand Junction branch would certainly be there as Kendall is in a no-mans-land for commuter rail users. (Especially if the Worcester Line is built for better speeds and service; there's no reason that the grade-separated line shouldn't have higher track speeds and Framingham-Back Bay travel times, express, in the 15 or 20 minute range.) The Kendall area is currently relatively easy to reach from the western suburbs by car (the only kink is the perpetual backups getting on and off the Pike at the Allston-Brighton interchange and navigating to Memorial Drive) but tedious by train, going all the way to South Station and then across downtown by Red Line. Direct service would certainly be appealing. In addition, there is a shuttle bus, EZ-Ride which runs, at capacity, every 8 minutes from North Station to Kendall but is prone to getting stuck in traffic; easy transfers and quicker travel times at North Station would be a boon to north-side commuters. With a new station in Brighton and new development there, it would be a good cross-town link. And better service in the Newtons, which currently see no reverse commute trains owing to bare-bones, 1960s-era single platforms, would be a ridership draw as well.
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Re: The Cambridge Tunnel

Postby MBTA3247 » Thu Jul 26, 2012 8:59 pm

ariof wrote:and to use it in the future as part of a South Station-Back Bay-Kenmore-Kendall-North Station (and perhaps one day back to South Station) loop. This would require a branch off the bridge over the Charles and under the Mass Pike to tie in below the existing Worcester Line which would be difficult but not unfeasible.

If my memory of the relative altitudes of the various roads and rails in the area is correct, the only way to make a loop connection work there with reasonable grades would be to have it split off the Grand Junction somewhere in Cambridge, tunnel underneath the Charles, then turn eastwards and come up between the Worcester Line and Mountfort St. The alternative would reach a suitable point below grade somewhere in Beacon Park Yard, then have to curve around under a good chunk of Allston and the western part of BU's campus to connect back to the Worcester Line. Either way is going to be a multi-billion-dollar project.

Such a route makes no sense for commuter rail trains. There would be no end-to-end ridership (Red to Orange will always be faster for SS to NS riders), just end-to-intermediate-points or intermediate-points-to-intermediate-points, which demands frequent stops that only a rapid-transit line can properly serve.
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Re: The Cambridge Tunnel

Postby Charliemta » Fri Jul 27, 2012 12:42 am

MBTA3247 wrote:The Cambridge Tunnel is only just below the surface. Given the amount of disruption involved, it would likely be easier to send a relocated Grand Junction branch below the Red Line, rather than digging up both routes. Of course, that would require some relatively steep grades, especially on the north end of the tunnel approaching Cambridge St.


Why not just eliminate Main Street where it crosses the Grand Junction RR? Block off the Main Streeet crossing and have the traffic go on Broadway, Portland St. and Vassar Street. Then there would be no need to deepen the Red Line tunnel of the Grand Junction RR at this location.
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Re: The Cambridge Tunnel

Postby MBTA3247 » Fri Jul 27, 2012 9:47 pm

You forget, there's still five other grade crossings in the area - including Broadway. In most cases, closing them isn't practical: doing so would just make traffic on neighboring roads even worse, and traffic in Cambridge is bad enough already per the design specs of Cambridge's public works department.
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