Over on r/mbta people were speculating: if Boston's rapid transit lines were numbered, like some other cities, which line would get the honor of being first? The Orange Line, as the main line of the historical Boston Elevated Railway? The Red Line, with the highest (pre-covid) ridership? The Green Line, as the oldest line (and the oldest subway in North America)? The Blue Line, as, uh, the only line to run cars that switch between catenary and third rail?

It turns out there's an actual answer! From 1964 to 1966, before switching to colors in 1967, the MTA (and then the MBTA) used numbers. Here's the 1966 system map:

The lines were:

1. Harvard to Ashmont (Red Line, Ashmont Branch)
2. Everett to Forest Hills (Orange Line)
3. Bowdoin to Wonderland (Blue Line)
4. Lechmere to Riverside (Green Line, D)

(I originally noticed the claim that they used to be numbered on Wikipedia, but then when I went looking for maps I couldn't find any examples and marked it as citation needed. Then I found that the parallel claim on the Blue Line cited the (incredible!) Changes to Transit Service in the MBTA district, which has this on p8. I realized that I'd been looking at maps that were too early, and when I checked the years immediately preceding the introduction of the colors I found it. I've now updated the page with a citation to the 1966 map.)