I did a survey of a number of J30's here in Annapolis several years ago. About 10% of the boats have tracks that go to the deck and this, evidently, is original equipment.
The decision was that if it was original equipment you can keep it but you can not add one or lengthen the one you have. Personaly, I do not see the advantage. It is more trouble sliding the car up and down than just lifting the pole up and cliping it on.
Larry Christy
J30 Class Measurer