The tracks in the new elevated St. John's Park Terminal were stub-ended (and used mostly for LCL). Below is an old NYC photo taken in 1934 when the terminal was about to open.
I think by the time the line was cut back to Bank Street in the 1960s (a 1962 NYC ETT I have shows the end-of-track was already Bank Street) I think train service was very sporadic. I don't know how they turned locomotives (they were all bi-directional) but they may have just shoved south when they had a load for a consignee with a facing point siding switch. If it was a trailing point switch they could've gone engine-first. Back in the 1960s operating a drill with an engine on each end was pretty rare.
Bank Street is where the line went "through" the Bell Labs building and from photos it appears to be double-track. It was also within yard limits.
Btw, when it was a working railroad line it was known as the 30th Street Branch or the West Side Freight Line. I never heard the name High Line until the preservation effort began. None of the former New York Central employees I've asked ever heard it called that either.