One Last Trick up Penn Station's Sleeve

Over the past several weeks we have seen many of the strange physical characteristics that shape the way trains into and out of Penn Station operate, and how C Interlocking and JO Interlocking work to funnel trains coming off the number of platform tracks down to the four East River Tunnels.  Over in Queens, we saw how F Interlocking and HAROLD Interlocking take the trains in Queens, and send them off on their way to Sunnyside Yard, Long Island, or up to Connecticut and New England.

However, when one looks at how things work in Manhattan and then how things work in Queens, the pieces don't exactly line up.  More specifically, I pointed out that in HAROLD interlocking, the trains are segregated by direction with the westbound tracks to the north and the eastbound tracks to the south, while in Penn Station, the trains seem to be segregated by railroad with the LIRR trains taking the higher tracks and the NJTransit/Amtrak ones opting for the lower ones.

And, just yesterday, a commenter on the HAROLD Interlocking post caught on to the discrepancy.  One would think that if the north set of tracks in HAROLD interlocking is only used for westbound service, and the south set for eastbound service, how would the LIRR trains get out of Penn Station? From the track assignment numbers, it appears that about half of the LIRR tracks cannot access the East River Tunnels Lines 1 & 2 for the eastbound pathway.

But that is where the Pennsylvania Railroad had one more trick up their sleeve.   Shortly after the trains enter the four East River Tunnel lines in Queens, the two middle tubes, lines 2 and 3, cross over each other to allow Amtrak and NJTransit westbound trains to arrive in the lower half of the station and LIRR eastbounds to leave from the upper half while still being able to be sorted by direction in HAROLD.

This little trick, visualized in this schematic below, plays a crucial role in how Penn Station, HAROLD interlocking, and everything else works:

Few LIRR eastbound passengers realize the tunnel their train is in cross underneath another tunnel just before they poke out into the daylight again in Queens.  This little trick that was thought up in the days when the Pennsylvania Railroad were designing the tunnels, allows for the railroads to maintain their separate corners of Penn Station without causing a huge messy set of interfering moves within HAROLD interlocking.  In this quirk, Line 2 crosses over Line 3.  There are no at-grade connections, there is no interference from one to another, and you can barely tell what is going on (the curves in real life are much more subtle as this criss-cross happens as the tunnels turn slightly north into Queens, this diagram is dramatized to show the point).

This one last piece of the massive New York Terminal District puzzle links up the craziness that goes on in HAROLD and the craziness that goes on in C/JO and allows the trains to move as smoothly as they do through the whole area.  When you're processing some 1,000 or more train movements through this insanely busy puzzle of rail infrastructure, little things like two tunnels crossing make a huge difference in how many trains can be moved, and how conveniently they can be moved, through the station.