When Elaine and Kramer are on the Subway they are on the #6 train on their way back from Queens to Manhattan. The #6 train does not run in Queens. The #6 goes only between the Bronx and Manhattan and certainly does not stop at Queensboro Plaza. The cars on the #6 train do not have seats facing forward or backward, so it is possible that the interior shot was done inside of the N/W line. This is evident from the yellow color scheme of the train seats, unique to the N/W (the 6-train color is scheme is green). Also, the N/W cars have front and rear facing seats, whereas the 6-train does not.