New York City is about join the ranks of US cities making a very expensive bet on a very old form of transportation: the streetcar. Mayor Bill de Blasio will unveil a $2.5 billion plan Thursday to run an electric train along the waterfront between Astoria, Queens and Sunset Park, Brooklyn. Running about 16 miles in length, the Brooklyn Queens Connector (BQX) will be the one of the largest urban streetcar systems in the US, the mayor's office said.

"This is about equity and innovation," de Blasio said in a statement. "We are mapping brand new transit that will knit neighborhoods together and open up real opportunities for our people."

"This is about equity and innovation."

The mayor's office said it was motivated by the fact that over 40,000 residents in 13 public housing developments — or 10 percent of the city's public housing population — live along the Brooklyn-Queens waterfront with little access to public transportation. The growing number of new residential and commercial developments along the route are also seen as reason to build up transit options for the waterfront. But the idea also helps de Blasio lay claim to a big infrastructure idea that was more associated with his rival, Governor Andrew Cuomo, or his predecessor, former Mayor Michael Bloomberg.

Fortunately for de Blasio, the city will not need the state's permission to begin planning for the BQX. (Cuomo has made it a habit of dashing the mayor's hopes and dreams.) Still, the streetcar is a long way off from reality: planning is expected to last three years, will construction will take four more, putting the BQX's targeted opening date sometime in 2023.

It's unclear why de Blasio is opting to build a streetcar — a hefty capital investment — rather than asking the Metropolitan Transportation Authority to add dedicated bus routes along the waterfront. The MTA touts Bus Rapid Transit a more cost-effective method of addressing the rapid increase in ridership. As of December 2015, there are 10 Select Bus Routes routes along nine corridors, with three more planned in Queens and Brooklyn. A spokesperson for the mayor did not respond to a question about the decision to support a streetcar over SBS.

Brooklyn QueensConnectorMap (1)

Streetcars have been around for a century, but the idea has picked up steam in recent years, as more and more cities turn to the throwback transit system to enhance their images as liveable. But they are often fraught with their own set of challenges. The Massachusetts transportation board is weighing whether to kill the Green Line streetcar extension in Boston, after cost estimates revealed the project to be $1 billion over budget. And the M-1 Rail project in Detroit, slated to open in 2017, has already drawn critics using the dreaded "B" word: boondoggle.

Which is not to say de Blasio's streetcar will hit the same speed bumps. The mayor's office says the BQX will operate "primarily" in its own dedicated lane of traffic to avoid conflicts with general traffic. Streetcars without dedicated lanes often run not much faster than a city bus, and unlike a bus, a streetcar cannot shift out of its lane to avoid obstacles in its path. That said, the proposed streetcar will average a speed of 11.3 mph, which is less than half the citywide speed limit of 25 mph.