F Train (240 person capacty per car) AND Tram (with a capacity of 110 passenegers per car).
There is only one tram car in each direction and it leaves approximately every 10-20 minutes - it would be extremely impractical to transport thousands. Not to mention the already refernced lack of parking.
Randall's Island, which is a pain to get to on public transportation is actually more practical (Icahn Stadium and many fields) - and that is impractical also since there is no train and 15,000 ppl (probably 10,000 who have been drinking) trying to take a bus seems like a nightmare.