Hmm, possibly. MA 128 is 57 miles long according to Wikipedia. The longest subway route in NYC is 32 miles long according to Google. I don't think the OP did the entire length of 128, since it cuts over to Salem instead of going to Rockport. So it might be a stretch to make the Yellow Line truly subway like (5-10 minute headways) but there's a number of Commuter Rail Lines that are that long (Fitchburg Line is 54 miles long apparently) that have hour or so headways. I imagine you could get 20 minute headways without totally breaking the bank, which might be useful enough for some folks.

I agree it's not a great investment, but in the Grand scheme of things we spend billions on highway maintenance each year (last I saw, something like $1.5B through MassDOT) so if we wanted to divert some of those resources, or increase taxes to fund more transit, at some point a Yellow Line would make some sense. But not until we've gotten the inner line running, and all the various extensions to 128 in the first place. Even then I don't know if running rails would make a ton of sense. Rapid Bus service might make more sense and be a lot cheaper. Just take 1 lane on 128, make it a bus lane, add some cameras/cops for enforcement, and run buses every 15 minutes. That would probably be totally affordable, especially since bus stations are dirt cheap and buses aren't terribly expensive.