Pretty cool —- should do it for NYC subways!
Boston T would be a better one as each of the colour lines are significantly different from each other, especially concerning green line trolleys. Even having not lived there for a number of decades I could probably still pick out at least red and green line. I might struggle to pick apart orange and blue line from each other as they are pretty similar trains, but I never spent significant time on that line... (My dad was a complete train nut and spent much of his spare time audio recording train rides around the world and when we lived in Boston, the local subway got the bulk of his attention. Here in the UK his hobby even got picked up by various TV companies and he got brought onto various talk shows to demonstrate his "Blind trainspotting" prowess by identifying various trains from their sound. All a ruse of course but it was a fun gimmick for a couple of years.)
So that's a change from 5 stocks (two A variants, two C variants and a D) across four lines to two (S7 and S8) in terms of the rolling stock. The deep tube lines will all get variations on the 2024 stock, likely in 2027 although I believe the announced date still clings to "late 2026" but for now are all distinct.
I think that's a bit optimistic. Right now, the only thing that has confirmed orders is the replacement for the 1973 stock (Piccadilly line). The same order also has options for further trains which would cover the Bakerloo, Central and Waterloo & City lines, but somebody still needs to come up with the money for it.
For the Bakerloo line trains (1972 stock) that's probably going to happen, since those trains really are getting long in the tooth now, but for the Central/W&C line stock (1992 stock) there's currently a refurbishment programme underway, so depending on how that goes, those trains will probably continue running for a while further.
That still leaves the Northern and Jubilee lines (1995 and 1996 stock respectively), whose replacement trains, whenever they might happen, will probably need a new tender – it could be that whatever train gets selected then will be a close relative to the 2024 stock, but I don't think it's automatically a given.
And the Victoria line – that one only got new trains in 2009, so those will continue for quite a while further and will be the last ones due for replacement on the deep tubes.
Chicago's "El" and subway system AFAIK, uses the same rolling stock on all lines though there may be a couple of different generations of cars in service.