[Mad_Nick]

If by "largest and most extensive mass transit system in the world", he is referring only to the subway, then yes, the NYC Subway has more track-miles than any other subway in the world, largely due to the widespread use of express tracks in the subway.
But "mass transit system" generally refers to the entire system of public transportation in a city, so in New York, this would include the LIRR, MNRR, NJT, PATH, SIR, and the various bus systems in the area. Here, Tokyo is probably the largest by far with extensive subway as well as a very extensive above ground rail network. And London's is probably larger, with its extensive Tube network as well as its South East rail network, and perhaps even Paris with its Metro, RER as well as Transilien commuter trains.