Hoo boy, I found it. God save me now.
https://drive.google.com/open?id=1X0jMoxMLl71me0nkItaclxa-NTezyaKD&usp=sharing
Wowzer some of these routes are out of hand. The 3 is a doozy -- Chelsea-Charlestown-Harvard-Watertown-Newtonville-Cleveland Circle-Longwood-Huntington-Back Bay-Seaport -- what?
And I do remember working on the 15, which was a classic case of "well, if it's gonna go here, it might as well go one stop further" being repeated ad absurdum: Airport-Eastie-Navy Yard-Sullivan-Union-Harvard-Allston-Coolidge Corner-Brookline Village-Jackson Square-Egleston-Ashmont-Milton-Mattapan-Roslindale Village-West Roxbury-Needham Junction-Newton Highlands. (Imagine how terrible that would be for tourists: "Hi hello, we're in Mission Hill and we want to get to Harvard. How do we get there?" "Oh yeah, you just want to take the 15 towards the Airport." "Wait, but isn't the airport in the other direction?" "Yup! Welcome to Boston!" On the other hand, how many tourists have been befuddled by our current inbound/outbound mess?)
I think one idea I was also playing around with was, "Our access to downtown is limited by our competitor, what is the closest we can get?", which I think is what contributed to the routes of the 7, 17 and possibly the 3, 9 and 10.
Ah yes, one other bit of context: this map did assume that the mainline railroads existed basically as they did in the real-world in the 1890s, which is why you see clusters at Back Bay and South Station, and why you see -- out in the suburbs -- some of the familiar ROWs being taken on by rapid transit. (I think a surviving mainline terminal in the Seaport was something I "handwaved" to explain the services there --
the New York and New England did run tracks out that way, so if you squint, you could kinda see it.) The 14 riffs on the idea of mainline networks that are converted wholecloth into rapid transit networks -- think of the Met in London. Just like the real-world Met, the 14 is a little different than the other modern-day lines -- more branching, terminates in the city, and extends "beyond" the traditional limits of the metropolis -- beyond 128 all the way to Brockton, just as Amersham actually sits in Buckinghamshire outside the M25.