Went on a mind thought train and categorized transit routes into 2 categories.
A: All dedicated transitways, these can be rail tunnels, rail stub tracks, dedicated busway tunnels, or abandoned railways. Subway tunnels are differentiated from dedicated ground level rail lines that funnel from North Station, South Station, or the BRB&L terminals. This includes the stub track and the SL transitway in South Boston.
B: All street running routes and on street streetcar routes
. This means the B, C, and E streetcar trolley lines are categorized with the buses, west of Kenmore, outside the tunnels.
Categorizing this way, this makes it easy to see where rail routes must define rail replacement shuttle bus routes, separate from bus routes and streetcar/trolley lines.
Inner boston core transitways and railways current/historical/abandoned, plus the rapid transit branches to Braintree via Quincy Center, and Riverside. I've also added the Watertown Branch extension to Waltham Center, even though it's outside of the inner core.
View attachment 34164
From this map, without using the North-South Rail Link, all rail lines on this map, are able to be fed directly into the subway tunnels past Downtown and through-run, on the other side, with the exception of the Fairmount line.
All northern routes are able to be fed into the Orange Line tunnel, with the exception of the Blue Line, and the B&M East Boston Line along Route 1A in East Boston. The Minuteman Branch can be fed into the Red Line tunnel. All branches that pass through the core of Somerville (E. Somerville/Union Sq) can also be fed into the Green Line tunnel.
All southern routes are able to be fed into the Orange Line tunnel, with the exception of routes that can be fed into the Red Line tunnel, and the Fairmount Line. The Highland Branch can be fed into the Green Line tunnel, but historically connected to the ROW that can feed into the Orange Line tunnel, so we'll say it can feed into both GL and OL tunnels.
Here are some of my thoughts on connecting existing transit and rail lines to form rapid transit lines, before adding new lines or digging new tunnels:
1. The Red Line tunnel will add a connection to Waverley. Braintree trains would be assigned to run to Waverley, as it could potentially be extended to Waltham Center, and Braintree trains serve Quincy Center. Ashmont trains will travel to Arlington Heights Busway. The Mattapan via Neponset line will be sent to terminate in the Seaport district via the Convention Center.
2. A rapid transit line would run from Waverley, to Davis, to East Somerville, then loop northwards to continue to Wellington to Linden Sq. This provides a one seat ride between Gilman Sq. and Davis. This is a crosstown connection line.
3. A rapid transit line from Newton Corner, then use the entire Grand Junction ROW. This is a crosstown connection line.
4. Aside from those two lines, Wellington Station would see the Medford Sq. Branch and Oak Grove Branch travel into downtown. The main corridor for Linden Sq. and the Chelsea-Revere line would travel to downtown via Everett. So 2 branches on each corridor.
5. For the Green Line, the Medford Branch would be extended to West Medford. There are now only 3 branches, as I removed street running service from the Green Line central subway tunnel due to reliability issues of street running traffic. Watertown Branch trains will be assigned to run through the Pleasant St. portal. West Medford trains will run through the Symphony portal (tunnel extension to cover the rest of the E branch). Arlington Heights trains will run via Porter and Union Sq., and continue through Kenmore (service continues on the D branch through Reservior).
6. On the south side, since the Medford Sq. and Oak Grove Branches are shorter, they will be assigned to run the longer routes to Dedham Mall and Wolcott Sq. Readville. Oak Grove trains will continue due south to Readville, Medford Sq. trains will curve westwards to Dedham Mall.
7. Linden Sq. via Everett trains will travel to Newton Corner, Chelsea-Revere trains will travel to West Roxbury VA Hospital.
8. The Orange Line tunnel is already responsible for now carrying 4 branches. However, there are currently no direct one seat service between Community College to any of the Somerville based rapid transit routes, even though there are 2 potential routings to reach Somerville from Community College. Charlestown riders must travel to Sullivan or North Station to transfer to a Somerville based route.
The Blue Line is mostly self contained, aside from extensions to Charles MGH, and potential extension past Revere/Revere St. to Lynn, which would continue to Salem Depot and Beverly. The B&M East Boston branch is mostly industrial along Route 1A, so unfortunately, I'm not sure if that line holds much value for connection. It does bring East Boston riders closer into the core portion of Revere, as opposed to Revere Beach, which is slightly further away from the core of Revere than the B&M East Boston branch. For now, it could be fed into the Blue Line tunnel.
The Fairmount Line is currently not connected and currently dead ends at South Station. The same is true for the Seaport Transitway.
Anyhow, these are my thoughts. I've left the map blank, since there's a lot of permutations on how to connect each line. The map here only extends to the extent of streetcar suburbs, as further out, the permutations for branches increases dramatically. Some ideas of note:
1. Use as many historical/existing lines as possible. There is no need to: use every junction in areas with more than 1 junction; or to use every dead end line in East Boston or South Boston.
2. Consider disconnecting the Green Line tunnel from all street running services, and only serve lines that are not street running. You may convert the Green Line tunnel to heavy rail if desired. This is due to reliability issues with street traffic, and the 4 street running branches decreasing capacity in the central tunnel.
3. Extra non-existant ROWs or tunnels that aren't current or abandoned are mostly not in scope of this. There are potential extensions of the Green Line subway tunnel from Symphony to Brigham Circle to Arborway, or Pleasant Street to Nubian to LMA/Forest Hills. Aside from that, the scope here is to reuse as many existing and former rail lines, and to minimize new tunnel/ROW digging. The idea here is the best way to optimize through running existing/historical ROWs. New metro tunnels could be dug under existing bus routes, so new tunnels would mostly be identifying Key Bus routes to dig subway tunnels underneath them.