Doesn't NYC mostly (mostly) use the same trains across the network? on the tube, each line was (historically) operated by a different train company, so most lines have a (somewhat) different profile but dedicated rolling stock to each line, along with different aged stock dependent on the procurement cycle or even age of the line itself.

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.)