The loud applause heard when the subway goes into the tunnel is because there had been prior shows which featured relatively poor simulations of an actual subway, and the audience was amazed with the effect the show's art department had managed to achieve.
Series creator, Garry Marshall, was a native New Yorker, and although series' co-star, Tony Randall, was born in rural Oklahoma, his long-time permanent home was the city, and he was a vocal supporter of it and its many offerings, including the NYC parks, and most well-known, opera. In fact, the second building used as Felix's and Oscar's apartment building, the San Remo on Central Park West, was Randall's long-time residence.
Felix and Oscar get on the 8th Ave local (known at the time as the 'AA'; it's now the 'C'), which is the correct line for their 'second' series apartment location, the San Remo, on Central Park West between 74th and 75th Streets.
Two years after this episode, Tony Randall and Barney Martin would co-star in The Tony Randall Show (1976).
In this episode, the "scatman" scats! Scatman Crothers plays the blind man.
