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The city may have to build new bridges across the Gowanus Canal and Newtown Creek to accommodate the streetcar system Mayor DeBlasio wants to construct along the borough’s waterfront, officials revealed on Friday.

Planners factored in funds for the brand-new spans in the trolley’s projected $2.5-billion price tag in case they can’t run tracks across existing bridges, Deputy Mayor Alicia Glen told reporters during a press briefing touting the scheme.

“We took a very conservative approach and ensured even new bridges could be accommodated,” said mayoral spokesman Wiley Norvell after the meeting, at which journalists were instructed not quote anyone verbatim.

The city still hasn’t announced the tram’s exact route from Sunset Park to Queens, but it will have to traverse both toxic waterways while coming in and out of Red Hook and Greenpoint.

Press materials show a mock-up of tracks going over Greenpoint’s Pulaski Bridge — where the city struggled for years to balance the weight of new bike paths with the drawbridge mechanism — but Glen acknowledged that may not actually be possible.

The new crossings could include bike and pedestrian paths, she added.

In the weeks since the mayor announced his so-called Brooklyn-Queens Connector project, critics have slammed the scheme as a boondoggle designed for the real-estate industry rather than to move people around efficiently, and officials attempted some damage control at the meeting.

They claimed the streetcar would have “right of way” for 75- to 80-percent of the path — pushing back against skeptics who are predicting the system will be a glorified bus on rails that will get bogged down in traffic.

They also encouraged reporters to reject the characterization of the streetcar — a plan first created by businesses and developers along the proposed route — as a service designed for yuppies, stressing the 40,000 public-housing residents who live along the path and the connections to transit-starved industrial hubs such as the Navy Yard and Brooklyn Army Terminal. The city claims 50,000 straphangers will ride the rails once the system is up and running some time after 2024.

But the honchos could offer few assurances on fears the tram’s ticketing system will not work in concert with subways and buses. They estimated half the streetcar’s riders will use it in conjunction with the state-run system, but couldn’t guarantee any free transfers for riders switching between modes on their daily commutes — echoing the mayor’s recent admission that discussions with the Metropolitan Transportation Authority are in their early days.