See that third center track that will allow Q trains to stay out of the way of through service once it begins on the Second Avenue line? That’s right, you don’t. Colin D. Young/Alamy

The Second Avenue Subway may not be finished in our lifetime, but even if it is, it will be stuck with reduced capacity. Thanks to a crucial decision made by the MTA back in the project’s first phase of construction, according to a newly published Regional Plan Association report, the new line will never carry as many passengers as it might have unless the MTA spends billions of dollars to fix a key problem.

The report, which concludes a two-year investigation into the MTA’s capital project overruns, notes that the MTA decided to reduce the number of tracks at the 72nd Street station, where the Q train arrives on the Second Avenue line, from three to two, while the number of platforms was reduced from two to one. This, the RPA says, will have a cascading effect on service, reducing the number of trains that can be run on the Second Avenue line and the Q once future phases of the project are complete.

The original plan for the station at 72nd Street and Second Avenue called for two “island” platforms with a track on either side and one down the middle. Once the T — the yet-to-be-launched portion of the SAS on Second Avenue south of 72nd — entered service, the center track would act as a terminus for Q trains, while T trains would use the side tracks. Passengers would have a cross-platform transfer between the Q and T in both directions, and each train would run on its own tracks with almost no overlap.

But all that changed when the MTA decided to scale back the 72nd Street station to two tracks and one platform, the way it is today. As a result, once the southern section of the Second Avenue Subway opens, the Q will likely have to run all the way up to the future terminus of the SAS at 125th Street, running an alternating schedule with the T, even though the lines will only share six stops.

The only other option would be for the Q to switch to the downtown track as it approached 72nd Street, wait in the station until it offloaded all passengers, and then go back the way it came. But in order to do that, all downtown T trains would have to be held up until the Q can get going again. Rich Barone, one of the authors of the RPA report, doesn’t see this as a real solution, because the Q would be “literally in the way” of normal T service.

The consequences of this decision will only be felt once the T begins service after phase three of the Second Avenue Subway project is done, which is likely decades down the road. But once (if?) that happens, Barone says the T will only be able to operate every six or seven minutes at peak hours, hardly the frequency expected from a showcase capital project. For comparison, the 4/5 currently has a maximum capacity of a train approximately every two and a half minutes, and subway lines in systems with modern signaling system can operate with a train every ninety seconds.

As Barone points out, reduced capacity thanks to subway lines sharing track is the problem with “our subway system in a nutshell.” For example, since the A is restricted at multiple points along its line where it shares tracks with the C and D, even during rush hour the line can only run every six to fifteen minutes with good service. The T will be restricted to similar headways.

Why did the MTA do this? The transit authority did not respond to a request for comment before publication. But according to Barone — who spoke to the MTA several times during the two years he researched the report — the authority claims it had to reduce the width of the station so it remained under the Second Avenue roadbed without spreading underneath adjacent buildings, which would have caused “geotechnical” concerns, as Barone puts it. He says the MTA was also worried about getting the necessary easements on those properties as well as community pushback to the increased construction, all problems that the report notes could have been eased by “greater city involvement” during the design phase.

The MTA originally denied to Barone that the scaled-back station would have any impact on service. But when Barone gave a detailed analysis of his findings and asked the MTA to essentially tell him why he was wrong, he says, the authority’s response “was more like a punt” than an explanation; it neither confirmed nor denied whether Barone’s assumptions were correct. He characterized the MTA’s non-answer as a “red flag.”

If Barone is correct, this means the four-phased project has already left lots of future capacity on the table, and there isn’t much anyone can do about it. There is no feasible way to expand the station now that construction is done, or to provide the third track necessary for Q trains to dwell without being in the T’s way.

Now the challenge will be to find a way to provide more frequent downtown service on the Second Avenue line for future New Yorkers. Barone offers the idea of swinging the 63rd Street line — which currently serves the F — southbound at Second Avenue, but because the F also shares lots of Queens track with the E, one of the most crowded lines in the system, it has no capacity to spare. Other potential solutions would require billions of dollars’ worth of construction, far more than it likely would have cost to find a way to complete the 72nd Street station as originally planned.

“The reality is, it’s done,” Barone concludes. “We can’t change it. We have to figure out how to work with it as it is.” Barely a year after it opened, the Second Avenue Subway is already just another subway problem to be solved.