Re: South Station Tower
Thanks! Yep, i'm more confused than evaar about what actually lies beneath or adjacent to i-93. If there truly is infrastructure in place that leads to a shorter completion time and cost savings - this is big news to me. With this wiki crap, not telling the story, i guess i've got plenty of reading to do.
It's not a pre-existing cavern. What they did for the Big Dig was drive the slurry walls that frame the 93N tunnel ultra-ultra deep, as that was the only way to build it in soft soil. And then they re-packed it underneath with clean loose fill that has no utilities and no rocks or other crap in it. The serendipitous upside of that is that the side pilings and the floor of the highway happened to frame 3 sides of a lower-level box with just the right dimensions to run trains through. So between the insertion point at Rowes Wharf and the departing point at Causeway Street all they have to do is scoop out all that loose fill, install a floor at the very bottom, install a center divider wall to hold up the highway (since the load-bearing fill will be gone), and pour a second waterproofing layer on the side walls and ceiling (which the highway upstairs lacks, but the narrower train ROW can have so it doesn't leak like the rest of the CA/T). End result is two 2-track bores running directly underneath 93N for three-quarter mile, and no surface impact whatsoever to construct it.
North Station Under is pretty simple: slips off the highway alignment where 93 starts inclining up to the surface, runs runder the east service driveway of the Garden, and plunks a 6-8 track/3-4 platform underground station directly underneath the above-ground platforms. Then crosses under the river directly under the current tracks and portals-up about 500 ft. north of Gilmore Bridge. Fitchburg and Lowell/Eastern Route/Western Route portals will split on the last couple hundred feet of incline because earliest possible portal spot *just* miss the corner of Boston Engine Terminal where the lines are split by the maint facility's southern 'wedge'.
South Station is a lot more complicated. The 1-mile lead tunnels from the NEC/Worcester and Old Colony/Fairmount converge directly underneath the current surface track merge, and spend their whole time right under the surface tracks. NEC/Worcester portal is at Washington St, Old Colony/Fairmount have forked shallow-level portals on the last couple hundred feet of tunnel because--just like on the northside--the northern 'wedge' of Amtrak Southampton Yard *just* misses the earliest possible portal opportunity. At SS the main tunnel swings over to Dot Ave. at the Pike vent building and straddles Dot Ave. and the edge of the Channel (
IMPORTANT!: This is how they structurally avoid any/all building impacts to the air rights tower pilings and the new row of Dot Ave. buildings coming post-USPS). Matching 6-8 tracks/3-4 platforms with North Station. Then it keeps going under Dot Ave., makes a little swing-out about 150 ft. into the Channel, and slices diagonally between the Seaport Blvd. and Northern Ave. bridges to insert underneath 93N.
Maybe there's a Central Station under Blue Line Aquarium if the portion under 93 is hollowed-out here into a wider cavern. But it would be very constrained and have fewer platforms, so it's dubious upside for the very high cost and a likely first cut from the project.
Electric-only. Steep grades, so best works on EMU's.
Will be able to take conventional locomotive-hauled commuter rail push-pulls from any and all diesel lines on the system if the T buys the same kind of
dual-mode locos that NJ Transit uses to get its diesel lines into Penn Station. The push-pulls would move a lot slower in the tunnel than the EMU's, so you'd want to up-front electrify enough of the system that most tunnel traffic is EMU, only the highest-demand diesel routes run thru, and the surface terminals handle the bulk of the traffic from low-frequency branchlines that don't merit electrification and heavy freight-traffic routes (Haverhill, for instance) that don't have the vertical clearances for tall freight cars under wires to feasibly electrify at all. One of several reasons why the surface terminals will always, always be fully utilized.
^That's the gist of it.
Now--before people start rolling eyes at the off-topic diversions--back to our regularly-scheduled tower and SSX discussion. NSRL truly is a whole entirely unrelated ball of wax decades apart that has nothing whatsoever to do with this tower or SSX.
