JAY interlocking, the brother of HALL interlocking just to the east of Jamaica, could very likely be called one of the LIRR's craziest interlockings. JAY is very much like HALL interlocking, but squished into a much smaller place and with some added twists.
First and foremost, below is the diagram of the whacky JAY interlocking (click to enlarge)
So you can see that JAY interlocking is quite messy. There's lots of diamond crossovers darting back and forth and lots and lots of steel and wire.
Here's what part of JAY interlocking looks like from the front of a M7:
So you can sort of get your bearings from this photo, this train is coming off of track 2. If you were to go straight on you would bear slightly to the right and head off to Penn Station. Towards the left of the photo there are six underpasses. From left to right, the first four are Atlantic Branch tracks back to Atlantic Terminal, the fifth is the Montauk Branch (the Lower Montauk branch) and the sixth is the Richmond Hill Yard lead.
The two tracks going over all of those underpasses is the eastbound mainline tracks, which go over all the others here.
The basic premise of JAY interlocking is the same as HALL's. Basically, the key concept is that things leave from the center of the interlocking. Now JAY is not as nicely set up and symmetric as HALL, but the same general concept is still there.
The setup is very similar to HALL on the east side of Jamaica, something that is very important in the grand scheme of things. For example, in HALL interlocking, the Atlantic Branch tracks are the center-most two. Likewise here in JAY interlocking, the Atlantic Branch tracks are the center-most. This becomes very important later on.
Just like HALL interlocking, you can almost draw a line down the middle of the whole thing and trains going in each direction would be segregated. Just like HALL, trains going in different directions are segregated from each other. The paths of an eastbound Mainline train and a westbound Atlantic Branch train will never have to cross.
This speeds things up exponentially in and around Jamaica. If everything was just a plain flat interlocking the amount of trains the LIRR would be able to run through Jamaica would be substantially less.
The way the LIRR has JAY and HALL interlockings set up also dictates how Jamaica station is used. Ever notice how westbound trains passing through Jamaica typically open their doors on tracks 1, 2, or 3? And how eastbound trains typically use 6, 7, or 8?
When looking how the tracks are set up it's pretty clear why that phenomenon takes place. A train from Tracks 1 or 2 has a pretty straight shot through the interlocking to get to one of the two mainline tracks out to Penn Station. Conversely, a westbound train has a pretty straight shot into tracks 7 or 8. For the most part, a train can go from any track to just about any other track through the interlocking, but doing it this certain way makes things flow a lot easier.
And now that we've determined there are dominant direction flows though the Jamaica complex, the next step is to point out that there are also apparent service flows through Jamaica. Just like in the last paragraph, do you ever notice that at Jamaica, trains to Penn Station typically leave from Track 2, trains to Atlantic Terminal typically leave from track 3, and diesel trains that are terminating at Jamaica or going to Hunterspoint Avenue will leave from Track 1?
That also has something to do with the way JAY interlocking is set up. Take a quick look at that track map again. Notice how a Penn Station-bound train can leave track 2 and an Atlantic Terminal-bound train can leave track 3 and their paths won't cross (the NYP train leaves towards the purple tracks and the ATL train goes towards the tannish tracks)
If they set up the station that way two trains (one to NYP and one to ATL) can both arrive and leave at the exact same time without bumping into each other, and that's what makes the whole timed connection thing at Jamaica work.
If it was set up much differently, the LIRR wouldn't be able to have well-timed transfers at Jamaica and operating two different western terminals would be a whole lot harder.
In roughly two weeks time (when I get back from vacation) I'll have a bigger "tying things together" post and it will make things a whole lot clearer. Believe it or not, there's a reason why Far Rockaway trains usually go to Atlantic Terminal. But that post will be coming shortly.