A J train departs Marcy Avenue onto the “S” curve heading toward the Williamsburg Bridge. By forcing trains to slow down, the curve limits the bridge’s capacity to 24 trains per hour. David "Dee" Delgado for The Village Voice

The picture of just how bad the L train shutdown will be for commuters is getting a little bit clearer. And it’s looking even worse.

A key to the latest problem was raised during Wednesday night’s L train shutdown town hall meeting in Williamsburg, when a speaker named Sunny Ng stepped up to the microphone and asked what many current L riders have wondered: “I have concerns about how many more trains can fit on the Williamsburg Bridge.”

At first, NYC Transit president Andy Byford didn’t offer a specific number. “Rest assured, our intention is to utilize the J/M/Z lines,” he said. “We intend to use them to the max.” But another audience member shouted, “How many?! HOW MANY?” and Byford passed the question to Peter Cafiero, chief of operations planning, who relented: 24 trains per hour can travel over the Williamsburg Bridge.

But 24 trains per hour, it turns out, is the absolute best-case scenario. “They can only achieve that if everything runs perfectly,” a source familiar with the planning told the Voice, on condition of anonymity for fear of losing their job.

Sunny Ng, an L line commuter, expressed his concerns during the MTA’s tunnel reconstruction town hall Wednesday night at the Progress High School in Williamsburg. David "Dee" Delgado for Village Voice

MTA planning documents obtained by the Voice note that “operational strategies [are] needed to maximize throughput,” which could include stationing extra trains along the route to slot in if a train is delayed or goes out of service, plus ensuring that train operators know the route perfectly and achieve the absolute maximum speed allowable under the current signal system.

Yet these same planning documents show that even 24 trains per hour, if achieved, represents an increase of only three trains per hour over the current schedule. That’s enough to carry approximately 6,000 more riders per hour than currently possible. The L train, for comparison, carries almost four times that, or 24,100 riders per hour, across the East River. Considering the MTA expects a large proportion of displaced L riders — up to 80 percent by their estimates — to opt for subway alternatives, and a large portion of those riders to opt for the J/M/Z, it’s not clear how the J/M/Z can possibly carry all the riders that will be crowding its platforms.

The problem stems from a stretch of the J/M/Z tracks between the Marcy Avenue and Essex Street stations on either side of the Williamsburg Bridge. Because the track has “S” curves on each side, trains must slow down to ensure they don’t derail. Even if the speed limit on the bridge were raised from the current 25 miles per hour, traffic jams would still form at the curves, much like when a car driver guns the engine in the middle of a city block, then slows again when getting to a red light.

NYCT President Andy Byford speaks to the press during Wednesday’s tunnel town hall. David "Dee" Delgado for Village Voice

Unfortunately, there aren’t any other lines with extra capacity to pick up the slack. There are currently twenty L trains running under the East River per hour; during the shutdown, there will be the three extra J/M trains from Brooklyn, plus two more 7 trains (assuming its new signaling system is finished by year’s end despite being two years behind schedule) and three more M trains from Queens — but also two fewer R trains, in order to accommodate the extra M trains (the lines share track in Queens). The MTA will also be lengthening C trains, as MTA chairman Joe Lhota promised when he announced the Subway Action Plan in July, adding the equivalent of one and a half trains per hour. That comes to 12.5 fewer trains total, an overall reduction in capacity of approximately 25,000 riders per hour.

This figure represents a big problem: It’s more riders than the MTA predicts will seek non-subway alternatives during the shutdown, meaning some number of people — roughly somewhere between 2,000 and 4,000 people per hour*, based on the MTA’s capacity estimates combined with information from the planning documents obtained by the Voice — may not be able to board Manhattan-bound trains during peak periods. But these numbers are very much in flux; the service plans are still being finalized and it’s hard to predict how commuters will react to the shutdown with any precision.

The G, which according to the planning documents currently runs nine trains per hour during peak periods, will add the most capacity of all: three additional trains running the full route, plus three trains per hour running between Court Square and Bedford-Nostrand, where they will short-turn and head back north to Queens — and all G trains will be lengthened to eight cars, double their current length.

This will increase service between Williamsburg/Greenpoint and the E, M, and 7 trains. But it’s important to remember that the G is just a means to an end for displaced L riders; nearly all of them will then be looking to cross the river via another line. So while the G can reasonably cope with the increased capacity, it is the transfer points and the other lines that will not be able to add nearly as much capacity that will suffer the most.

The G line will run more and longer trains during the L train shutdown — including extra shuttle trains between Court Square and Bedford-Nostrand — but those riders will eventually have to transfer to other already-jammed lines to reach Manhattan. David "Dee" Delgado for The Village Voice

“It’s frustrating how secretive NYCT has been with the entire process,” the source lamented to the Voice before the information was revealed publicly at Wednesday’s town hall. “I’m not trying to make the MTA look bad, but the public deserves to know what’s going on.”

During the town hall, Cafiero stated the priority is to run as many M trains as possible, while also accommodating as many J trains as possible over the Williamsburg Bridge. If this holds true, it means the MTA will target fourteen M trains per hour and ten J trains — the most that can run under that 24 train-per-hour limit imposed by the “S” curves.

“It’s pretty much a given that the line will be over guideline,” the source told the Voice, meaning more people will be trying to pack on than the maximum capacity. “I don’t know what to say besides we’re fucked and it’ll be miserable.”

*The math goes like this: 24,100 riders per hour during rush hours currently cross the East River on the L. The MTA estimates that between 20 and 30 percent of them — between 4,820 and 7,230 — will get to work by buses, bikes, or other non-subway means. Counting the longer C trains, the MTA will be adding the equivalent of 7.5 new trains worth of capacity, enough to accommodate approximately 15,000 passengers. That would leave between 1,870 and 4,280 commuters unable to squeeze onto trains at all.