The A division is the IRT lines (numbered lines) and are 1 foot narrower and 9-24 feet shorter than their B division counterparts and have only three sets of doors per side rather than four sets per side. The IRT was built by August Belmont who wanted no freight service on “his railroad” and designed smaller clearances to ensure no freight trains. To this date, A and B division cars are separate and only A division cars are operable (ignoring the wide gap) on either A or B Division tracks and then only with adjustments or special procedures.

There are three interconnections between the A and B division :

• from the Jerome Avenue Line through Jerome and Concourse Yards to the Concourse line.

• Full diamond crossover at Queensboro Plaza (thanks to former dual BMT/IRT operation of Astoria line (Now BMT) and Corona Line (Now IRT)

•  From the New Lots Line center track at Junius Street to the Linden Yards and Canarsie Line (no Third rail)

Except for portions “inherited” from old elevated lines, the IRT is the oldest of the three subway divisions and most of those have been demolished or heavily rehabilitated.

Much of the IRT is built to IND/BMT specs. The exceptions are the Contract One and Two lines (the original subway lines) and the Steinway Tubes (original built for trolley service). The rest of the IRT (as well as much of the BMT) is part of what is known as the "Dual Contracts" lines, and most of the Dual Contract lines are built to BMT specs. Of course it must be remembered that the Dual Contract lines allowed for the specs of the standard of the BMT back then, which was 67 foot cars. Think of the Eastern Division Lines (J/M/Z/L). Those are basically unaltered BMT lines. In the 70’s, some of the BMT was altered to allow for 75 foot cars to run. So, when talking about "being built to BMT specs", I am referring to the old standard which would allow the current 60 foot cars to fit, but not the 75 foot cars that run on some lines (the system no loner runs 67 foot cars).

Below are the IRT lines that were built to BMT specs, and can fit 60 foot cars, if of course the station platforms were shaved off a bit (they are slightly wider to allow for the narrower IRT cars that run there). And also of course, let’s not forget that over time, some signal buildings, or other obstructions may have been built here and there that may have to be removed or altered, but the basic tunnel and el clearances themselves would allow for the wider BMT sized rolling stock.

• The Lexington Line north of Grand Central. (4/5/6)
• The Broadway-7th Ave Line (West Side IRT) south of Times Square (including Times Square, but excluding the South Ferry loop) . (1/2/3)
• The Dyre Ave Line and all the other Bronx IRT Els except the West Farms El south of East 180^th St.
• The Brooklyn IRT Line east of Atlantic Avenue, including both the Livonia El, and the Flatbush line. (2/3/4/5)
• The Clark St Tunnel from Manhattan to where it joins the Joralemon tunnel route. (2/3)
• The entire Flushing Line in Queens, excluding the Steinway tubes. (7)

• The Lexington Line south of Grand Central, including the South Ferry Loop (which originally was built for the Lexington Line, not the West Side line, as is the operation today). (4/5/6)
• The Broadway-7^th Avenue Line (West Side IRT) Line north of Times Square, excluding 42^nd St-Times Square itself (which is Dual Contracts). (1/2/3)
• The 42^nd Street Shuttle. (S)
• The Joralemon Tunnel and route from Manhattan to Atlantic Avenue in Brooklyn. (4/5)
• The Steinway Tubes on the Flushing Line. (7)
• The 2/3 Line from 96^th St to Lenox and including the West Farms El to East 180^th St.

Everything on the IRT not included in the second list is built to IND/BMT specs, and could technically carry 60 foot car. Of course the platforms would have to be shaved, and I'm sure their have been some encumbrances built since then (signal houses and the like), but the tunnels themselves are built to allow BMT sized cars to go through (possibly the length of a BMT Standard which was 67 feet, but certainly a 60 foot car).

Two photos of underwater redbirds courtesy of Rich Galliano