Media depictions are a hugely underrated aspect of public transport perception.
In japanese and korean media (my experience is with a ton of anime and k dramas, more in the former than the latter though) trains are very common casual and serious backdrops for a variety of scenes, either within the train, at the station, or just a train passing by on the bridge in the background.
In Hollywood/American TV, it's always cars, with the occasional airport/plane. It riles me up quite unreasonably that shows/movies set in New York fuckin city with 24-7 subway service, and characters are shown trying to catch a cab in Manhattan in the middle of the night to go 20 blocks away. At least Marvelous Mrs. Maisel acknowledged this directly as a class thing for some characters and other characters took the train, but most movies just assume the American viewer cannot relate to someone using the subway.
It's almost certainly less to do with location scouting and production design, and more about perceived biases. If studio executives think American audiences can't relate to trains or subways because they're less commonly used in real-life, we're going to see fewer of them on the screen. There's probably a cultural blind spot at play there, too; if writers don't ride trains--or even perceive them--very often, they're far less likely to write about them. It's a vicious circle.
As for locations, a quick Google search found a genuine New York City R17 subway car[0] that can be rented out in LA. The Sierra Northern Railway--a freight carrier in California--has rented out[1] its rolling stock, facilities, and tracks to film productions for nearly a century. They've got quite the roster, spanning multiple eras. There's also Amtrak, the various local/regional metro systems, other rental companies, and even private collectors if they need something specific.
As for stations, that's even easier. Various urban backlots have underground subway station entrances[2] where you could characters exiting the station. Or the station platform itself is just a long room; you don't have to show the actual tracks, or you could composite in a train moving across the frame, etc. Plenty of permanent sets can play that role. Set designers do far more with less all the time. Hell, you can just reference it off-screen for a sitcom. That's a huge chunk of Seinfeld (or any sitcom). Shit happens, everyone reacts...often poorly, with hilarious results.
A subway set is probably expensive, especially if you want a working train.
Almost everything set in London uses either the disused Aldwych Station on the Piccadilly Line, the disused Charing Cross station on the Jubilee Line, or the Waterloo and City line at weekends when it is normally closed — sometimes even when the setting ought to be a much larger train and style of station elsewhere.
Yeah, the technical aspects of the subway shots in Seinfeld are interesting, and really give a sense of how hard everybody worked to make that show what it was.
The first scenes to take place in a subway (to my knowledge, anyway) were in the 30th episode, which was called "The Subway". And indeed, most of the episode takes place in a subway.
The subway set for this show was rented from Warner Brothers and was sent to the Seinfeld lot in several pieces on trucks. It was apparently a huge PITA and presented a lot of technical limitations.
They assembled it on springs and had a bunch of crew shake the car to simulate movement. They had to light it using stage lights, manually simulate the mechanical opening of the doors, etc. And it was apparently difficult to do anything but a closeup without it looking fake.
They redecorated it for each of the different subways that it was used to depict (in the episode, different characters are simultaneously taking different subways to different places).
After the episode, the set was disassembled and placed back on the truck to send back to WB, but the truck driver went under a low underpass, which the set struck and was destroyed. As a result, Tom Azzari, Seinfeld's Production Designer, led a small team within the show to design, engineer, and build an entirely new subway set, fixing all the technical problems while they were fresh in the team's memory. They even got actual subway light fixtures and pneumatic doors. This set was used for the remainder of the show, and went on to be used in other shows and films as well.
(There are a few interviews about this process on the Seinfeld DVD extras, Season 03 "Inside Looks")
In japanese and korean media (my experience is with a ton of anime and k dramas, more in the former than the latter though) trains are very common casual and serious backdrops for a variety of scenes, either within the train, at the station, or just a train passing by on the bridge in the background.
In Hollywood/American TV, it's always cars, with the occasional airport/plane. It riles me up quite unreasonably that shows/movies set in New York fuckin city with 24-7 subway service, and characters are shown trying to catch a cab in Manhattan in the middle of the night to go 20 blocks away. At least Marvelous Mrs. Maisel acknowledged this directly as a class thing for some characters and other characters took the train, but most movies just assume the American viewer cannot relate to someone using the subway.