A thought exercise:
What happens to the frequent grid if all the mainline RR ROWs carried rapid transit service in the inner core?
Some of the frequent surface routes would be re-allocated onto other routes within the system. For example, the 32, 35, and 36 currently all duplicate rail corridors due to the lack of rapid transit in the south west of Boston. If West Roxbury and Hyde Park had rapid transit, the 32 and 36 would all be scrapped, and instead the 30 and the 34 become the high frequency routes.
A thought test of what a fantasy high frequency grid of transit routes in Boston would look like. I went through and did a rudimentary run of cannabalizing a handful of duplicated frequent routes from
the BNRD map, onto a new map that extends rapid transit; and re-allocating frequencies into other surface routes.
There are a few possible corridors not on this map that I wasn't so sure whether to add. Some corridors are Arlington - Medford Sq, the 119 in Revere, an Arlington - Mt. Auburn/Harvard crosstown, or a 92+95 combo route.
The map on the bottom right is a rough estimate of what Boston's frequent bus and subway network would look like if it had similar frequencies to some similarly sized city like Amsterdam. The rail corridors outside of the inner core are placeholders if some of them don't make any sense (those not covered by any MBTA routes are simply shaded as a very thin black line with no frequency marker, or I simply couldn't be bothered to include them). I simply tried testing what happens if every other train was through routed (essentially cut headway frequencies in half outside of each terminal). It was mostly an experiment/test, so outer core rail frequencies should not be taken at face value as actual "dream/fantasy headways".
For a comparison to today's pathetic service levels,
see here. I actually made a similar map (but I had no idea what I was doing at the time), back in mid 2022 after the original BNRD map was posted, but the small size of the subway system really destroys how many high frequency surface routes can exist, since buses have to cover the RLX to Arlington, BLX to Lynn, OLX to West Rox/Wyoming Hill, BLX/GLX to Waltham, BLX to Newton Corner, GLX to Porter, GLX to W. Medford, the existing RR ROW to Hyde Park, etc., 5 RL infills between JFK UMass and North Quincy, etc., etc., etc.
With rapid transit on all of those corridors, the frequent network would essentially shift to cover other parts of the network. Take a look at Lynn and Roslindale on this fantasy frequency map compared the frequency map of today.
I've left the Quincy/Waltham/Lynn/Newton terminals as is without rapid transit extensions to Braintree/Swampscott/W. Newton, so the frequencies on this fantasy map kinda leave room for improvements. It was intentional, since the surface routes still have to cycle back to the main city center terminal, instead of hitting the first rapid transit station. Essentially, my thought is that rapid transit extensions would work best if they can eat away at excess bus running. I'm still not sure how to find info whether demand is greater a grid based system or hub & spoke for these terminals. I can make a 2nd dream fantasy map adding more frequent corridors. if it is so desired.