And ironically, it doesn't even include all of Boston. I can think of at least 6 train stations within city limits that are Zone 1.
I think the better indicator than Zone 1A is the density of bus routes, and connectivity/continuity of urban development of the inner core. Or essentially neighborhoods covered by the old BERy streetcar network. Categorizing busable arterials into a map, the "inner core Boston area" is easier to see and mark as a rough circle of inner core development.
Zone 1A excludes much of southwestern Boston, although the Arboretum/Forest Hills parks kind of isolate West Roxbury/Roslindale from the rest of the city, focusing into a narrow corridor to connect the neighborhoods. The Commuter Rail zones don't really do a good job of describing distance/connectivity to Boston. IMO, the density of bus routes is the best indicator of whether it's "inner core Boston" (most of them are BERy routes). (i.e. a neighborhood's bus route isn't based out from Waltham/Lynn/Quincy/Salem, and doesn't feed from the outer suburbs (i.e. more of a "local" bus route, than a "suburb feeder" bus route). If so, then it's "inner core Boston", than "Outside of Boston".
Essentially it boils down to:
Transit Fare Zone 1A is a mapped thing, Here I am using it as basically Boston’s urbanized metropolitan core, even though its come to include Cambridge Somerville and some or all of the next ring of cities/towns…
or “where the streetcars ran” or “where the buses are useful” or “where Commuter Rail fare is same as Subway” or where Blue Bikes are offered or “where you can bike or walk for errands”
Fuzzy equivalence, but roughly true.
Or in map form, here's the area roughly being described.
I'm always struggling to figure out where the "core area of Boston" ends. Essentialy, areas that do not have a majority of area zoned for SFH, areas with good urban continuity, areas that make an attempt at mixed use zoning. Areas you can commute from Boston without hitting an extended area of nature reserves with no urban development at all. Areas not quite out to 128 (the edge of the buses), but make some of the way there from Boston (roughly the last "bus hub" before 128). Areas you can bike to in under an hour in more than 1 direction, areas you can bike to and stop somewhere along the way for an errand, places you can bike to for far more than just commuting to Boston. Neighborhoods with "at least a sidewalk going the whole way" on both sides of the street. Areas with a bus stop nearby and you can bike to another bus route's bus stop.