If your a regular (or semi-regular) Montauk Branch rider such as myself, and you take through trains to Jamaica, or further west, then you may not be able to help wonder why they sometimes take one route and some take another. This is a unique feature of the Montauk Branch that, in disaster, gives it one line of defense against extensive delays that could occur on either of it's possible Montauk Branch route options, the rest of Montauk Branch itself, or the Central Branch/Main Line. In an event of a disaster, Montauk Branch trains get the little known advantage of being able to "advert disaster" by choosing the other and completely bypass the chaos. It's rare that in the LIRR is there a time where you can take two routes to get somewhere, and taking about the same amount of time, either way.
Passengers like me who travel on the Montauk Branch east of Babylon get not only 1 or two routes, but three routes that their train can take. Westbound trains can either diverge just west of Babylon onto the Central Branch, taking them up to the Main Line, allowing them to flow through communities such as Westbury, Mineola, New Hyde Park, and through the eastern layer of Queens before coming into Jamaica, or, they can travel along the South Shore flowing through all those flowing communites, at near 80, if they were to travel this way, once they get to Valley Stream, they get two options to get them to Jamaica as they can either take the Montauk Branch (via St.Albans), which is the route they typically take, or if there's some sort of service disruption, they can take the Atlantic Branch, getting them into Jamaica around the same time. This gives Montauk Branch riders a secret advantage that wouldn't be seen on the other branches, as if there was issues on the Main Line, the other two diesel branches would be suffocated, and meanwhile, we'd be able to sail through.
With only a couple of exceptions, almost every Montauk Branch train can travel via the Main Line or Montauk Branch. With the exceptions being westbound trains 35, 37, and 7, and the eastbound ones being 76 and 80, all other eastbound trains can easily take either line. While generally speaking the LIRR plans out which trains take which, and trains typically stick to that order, in an event of a service disruption, track work, or some other planned/unplanned event, these trains could take either line. This allows the LIRR to avoid having to substitute. in other ways by giving them two options to get to Jamaica. This also gives them much more rush hour capacity as this allows them to have two options to run trains and typically given the third track, the LIRR tends to run the rush hour trains via the Main Line as there's more capacity up there (the only exception to this is train #5 which typically takes the Main Line, most likely due to the higher concentration of trains on the Main Line at that time.) I complied a document showing the train number and which route these Montauk Branch trains are scheduled to take on a normal day.
With things like a railroad, there's rarely as an always, but generally, these trains stick to these routes. Of the trains that deviate their scheduled routing, I'd say the most common one must be train 8 to Montauk, which of them, most commonly gets deviated as #8MTKXX and runs via the Montauk Branch. In terms of the train that most sticks to it's routing, that award almost definitely has to be given to the Cannonball, which has done a better job sticking to it's Main Line routing then some of the trains that stop on there in the first place!
I didn't include weekend trains, as I can't access their routes on a weekday, but generally, more of them then not take the Main Line. Still, there's a decent selection that take the Montauk Branch and even more so then on weekdays. I've noticed that more of the Sunday Hamptons trains have been taking the Main Line as last year only #6015 really did, but I guess this year they changed that as they become more comfortable with the third track. If you really dive in, you'll notice a couple of patterns, such as these trains taking the Montauk Branch at night in almost every case. My best guess for this is because generally taking the Montauk Branch is the better choice, it's slightly faster, there's no grade crossings involved, etc, etc, but during the day, given the high velocity of local trains on the Babylon Branch, it's difficult to get these trains up to speed when they could sail down the Main Line 3rd track.
