Does anyone know why one has to be on the first five subway cars to get to the South Ferry for the Statue of Liberty? Does the last cars seperate and branch off to somewhere else?

No separation...its just that the platform at South Ferry only accomodates the first five cars. The second five cars don't have their doors opened since there's no platform...just a wall.

The trick is knowing what car you are in. If you didn't count the cars when you got on, you'll sit there wondering when they keep telling you to move to the first five five cars if you happen to be in one of those or not.

This is true for the 1 and 9 trains that stop at the station called "South Ferry."

Other trains to Battery Park and the Statue:
Just across the street from South Ferry, the N and R stop at the "Whitehall" station. At the north end of Battery Park, the 4 and 5 stop at "Bowling Green". For all these trains you can ride in any car.

Once you venture all the way downtown and see how old and small these subway stations are...some are close to 90+ years old...then you will realize why you must be in the front 5 cars.

Same situation with the Long Island Rail Road when the trains stop at Forest Hills and Kew Gardens.