NJTransit doesn’t care if the event is public or private. They have an arrangement with the Meadowlands Complex to provide transportation based on the number of tickets issued.
The train is running so that point is moot, but Siyum tickets are very different than Football tickets.
The Siyum Organizers rented Met Life and dealt with the tickets themselves, as far as the meadowlands is concerned no tickets were sold.
For a Football game the tickets are sold on the open market via ticketmaster or NFL Ticket exachange so they know how many tickets are sold and approx how many people are going (There are always No-Shows)
The only way they could estimate how many people are going to the Siyum is the Agudah has to tell them and keep them updated on ticket sales