Over eighteen miles of the L&NE main was trackage rights over a track that essentially went nowhere on its own.
Not quite nowhere. At one time the NYS&W crossed the Delaware River over its own bridge and continued on to Wilkes-Barre, Pa. via its subsidiary, the Wilkes-Barre & Eastern. Anthracite for the New York City area was big business, but the NYS&W got into it too late in the game. The Pennsylvania trackage was first abandoned while the NYS&W still serviced Columbia, N.J., but the L&NE overhead traffic was what kept that portion of the NYS&W alive.
The interchange point at the western end of the NYS&W section enabled the NYS&W to get just a little bit more revenue from their portion of the car's travels. Actually, if the car was destined to go from the L&NE to the NYS&W, even if the L&NE handled the car between Hainesburg Jct. and Swartswood Jct., the NYS&W would still be entitled to its full share of the revenue, but then the L&NE would have a claim for doing the handling, and then the bookkeeping gets a whole bunch more complicated. For the bean counters, Hainesburg Jct. was easier to deal with.