It was an ambitious idea by Mayor Bill de Blasio and the focus of his 2016 State of the City speech: a sparkling streetcar running along the waterfront in Brooklyn and Queens.

But two years later, there has been little progress on one of Mr. de Blasio’s key transportation initiatives.

Mr. de Blasio has said little about the streetcar in recent months and his office has not identified a route, leading to persistent speculation that the proposal is dead. At the very least, it is delayed. Supporters now question whether a groundbreaking could happen before Mr. de Blasio leaves office at the end of 2021.

In a recent radio interview, Mr. de Blasio insisted that the streetcar was still on.

“We’re moving forward, but we have to get the exact details right,” the mayor said on WNYC on Friday, calling the proposal “a big complicated endeavor.”