Mayor de Blasio's developer-backed Brooklyn-Queens streetcar project may not pay for itself as he has promised it will, according to a draft memo prepared for Deputy Mayor Alicia Glen and obtained by reporters.

The 16-mile BQX streetcar line from Sunset Park to Astoria is premised on the idea that building the streetcar line will boost property values, prompt more construction, and create jobs along the line, which will in turn generate enough tax revenue to cover $2.5 billion in bonds used to build the thing. However, since de Blasio announced the project in 2016, development and tax experts have pointed out that this may not work, in part because real estate values along the northern Brooklyn and western Queens waterfronts have already skyrocketed without a streetcar, and are already largely served by transit. The city Economic Development Corporation's own study on the issue found that waterfront property values would rise with or without the BQX. The million (billion?)-dollar question is whether they would rise enough to cover the huge cost of the slow-moving trolley system.

The draft memo from the BQX Project Team to Glen, dated February 10th, cites "several serious challenges" with pulling off the streetcar, including, "Value capture not providing significant revenue to fund the entire project as originally stated." (Value capture is the idea of harnessing increased tax revenues.)

The memo also notes that the time and cost of relocating utilities along the route is the most expensive piece of the project, and "has the possibility to make the project unaffordable and render implementation timelines unfeasible."

The memo lays out three options in terms of how quickly to proceed with the project, including a "Red Light" approach that would put it off by a year and a half to further assess the details. It also suggests including in the environmental impact statement a look at whether it might make more sense to create a rail-free bus-rapid-transit route instead.

"We think that kicks the legs out from under the whole premise," Jon Orcutt, director of advocacy at Transit Center and a former Department of Transportation official, told the Daily News. "If these doubts bear out, the mayor should give us all a break and cancel the project."

The Mayor's Office said that's not all she wrote.

"This is as nitty-gritty a study as anyone can do," Mayor's Office spokeswoman Melissa Grace wrote in an emailed statement. "We’re costing out the price of moving specific water mains, and estimating tax revenue from individual lots along the route. The numbers change constantly, and that study has to be completed so we can move ahead. The project will improve transportation for hundreds of thousands of people, and we continue to work to move it forward."

The draft memo indicated that a projection of the tax base funding was supposed to be completed in February. Grace said the value-capture analysis is ongoing.

The memo states that the streetcar poses the "likely need for the use of some city capital," and will likely require an initial commitment of $58.5 million to get going next year, with ground being broken in 2019 under the quickest scenario. The front-end costs will be lower if the project is delayed, but the overall cost will be an additional $100 million for each year it's pushed back, according to the missive's authors.

The peculiar transportation project would be separate from the MTA system and parallel existing bus and subway routes, and MetroCard transfers are not guaranteed to be part of it. Over half of its route is in a floodplain, and nearly a quarter of it is in an area susceptible to more severe flooding due to storm surges, according to reporting by the Village Voice's handsome city editor Christopher Robbins.

The only way that the streetcar proposal makes some sense is if one considers that seven developers with projects along the proposed route contributed a total of $245,000 to de Blasio's nonprofit Campaign for One New York, which he shuttered last year during a federal investigations into possible play-to-play schemes. That investigation concluded in March without charges of criminal wrongdoing.