There are very few television shows set in New York that even include the subway system (Carrie Bradshaw & Co. seemed to be blissfully unaware of it). Seinfeld kept things reasonably real, however, and while the characters were often walking or driving fake Jon Voight's car, we do see them underground in several episodes—one even took place almost entirely in a train car.
When they did incorporate the city's mass transit system into the show, it was always the same subway set, which is still being used in other productions today.
The same subway car set was recycled for all the subway scenes in this episode. The crew merely decorated it with different posters to differentiate the scenes that featured the set. To create the illusion of a rocking subway ride, several stagehands stood on either side of the car's outside walls and rocked it back and forth with 2x4s. The set was on loan to NBC from another studio, and on the set's return to its original soundstage, the truck carrying the set accidentally destroyed it while traveling under an overpass that was too low. The set was rebuilt and is still used.
After coming across this photo earlier this week, we wondered about the large curved seats and the arm rests—were these ever part of the design? While the train car they created does look a little bit more realistic than others we've seen, it doesn't look exactly like anything that has ever existed here, not even in the 1990s, when the show was filmed.
The MTA's Adam Lisberg told us this week, "We never ran a car with seating like this. It’s kind of a cross between the R42 (which you can still find a few of on the J-Z) and the R46 (still a workhorse on the A and F lines). But the 'conversational seating' configuration on those is three-and-two (three parallel to the car body and two perpendicular to it), not two-and-two. And rails on the side of seats are usually next to car doors, not the aisle."
Wiki notes that there were some inaccuracies with the show's portrayal of the subway system, as well: "Several times the subway rollsigns indicate a 5 train bound for Utica Avenue, where it doesn't even go. The train, instead, branches off to Nostrand Avenue on weekdays. On weekends, it terminates at Bowling Green in Manhattan." The subway etiquette issues, however, were portrayed in a very real way—check out Kramer's manspread:
Here's the same car in a different episode: