The three Jamaica interlockings--DUNTON, HALL, and JAY all work together to smoothly get trains in and out of this key transfer station. DUNTON plays a lesser role, being off on the Atlantic Branch and all, but JAY and HALL are the really big ones.
Here is a real zoomed out track diagram of the Jamiaca area. the tracks that are a part of DUNTON interlocking are colored blue (look in the bottom, left corner). The tracks that are a part of JAY interlocking are colored red, and the parts of HALL interlocking, today's focus, are colored green:
Because JAY and HALL need to work together to get everything running smoothly it is very important that you read through the JAY interlocking walkthrough when it is posted in a few weeks. There will also be a post shortly after that tying everything together...that is a very important post in terms of understanding how the LIRR can pull this whole show off.
Now I'm not going to sit here and describe each and every switch--that wouldn't do you any good. Instead, I'm going to simplify this diagram and talk more about the theory and the concepts behind HALL instead of the actual switch arrangement. Once you grasp just how trains enter and leave HALL interlocking, the whole layout of the plant begins to make a huge amount of sense.
It will all become clear very shortly--trust me.
Here's a simplified diagram of what goes on in HALL:
Here's one of the most important sentences in the whole post: the key design feature in HALL interlocking is that the Montuak and Atlantic Branches join the interlocking from the MIDDLE of the interlocking.
Take a look at this photo:
|(Photo credit: Eric Haas/Red Over Yellow.com)|
Still don't see where the Atlantic Branch tracks duck out?
|(Photo credit: Dave Keller Archive)|
The same thing happens with the Montauk Branch. Just east of HALL the Montauk Branch tracks are the two middle ones. When it comes time for them to part ways with the mainline they rise all the way up and go over the two eastbound mainline tracks. Here's an old photo (and the only one I could scrounge up) from Trains Are Fun.com. You can see the train on the big viaduct that takes the Montauk Branch tracks from the middle of HALL interlocking and brings them out and south towards St. Albans. The photographer is standing next to the two mainline tracks, which you see in the foreground, while the train is on the big viaduct that leads off the Montauk Branch.
Now take a look at the Bing Maps Bird's Eye View of the area. Notice how the Atlantic Branch tracks leave from the middle. Then scroll towards the top of the screen and notice the Montauk Branch tracks leaving from the center over the Mainline tracks.
It is this whole middle thing that allows the LIRR to operate the huge number of trains they currently do. The way this interlocking is set up, eastbound and westbound trains will never have to cross paths. Eastbounds do their thing, westbounds do theirs.
You could draw an imaginary line down the middle of the interlocking and you would see that all of the eastbound tracks are on the one side, and all of the westbound tracks are on the other.
Note the yellow dashed line in the simplified diagram:
There are no crossing of tracks. Every time one of those lines in the simplified diagram above intersects, it is a grade-separated intersection. The Atlantic Branch tracks pass underneath everything else and the Montauk Branch tracks pass over everything else.
The dispatcher at HALL doesn't need to have a Mainline eastbound train wait for a Westbound Montauk Branch train to crossover the tracks. It just goes underneath it.
There is a second very important feature that has to deal with HALL interlocking, but unfortunately I am going to withhold that piece of information until we do the JAY interlocking walkthrough. After that there will be one big "tying things together" post that will trigger a lot of Eureka! moments for many. Hold on!
This interlocking walkthrough is very, very, basic. I have left out a lot of information that I thought would really confuse you. As long as you understand that one key concept (that the whole entering and leaving from the middle of the interlocking, thus there are very few conflicts), you are in pretty good shape, as that will come into play during the JAY interlocking walkthrough as well.
Pay close attention to the JAY interlocking walkthough, and compare it to this one. Trust me, it will all make sense in the end. Just give it time.