Of course, the announcement was also celebrated by New Yorkers who rely on the line. Jocelyn Crespo, 35, who lives in Williamsburg, Brooklyn, described herself as “very relieved.”

“It gives us more options to get into work and opportunities to get to our friends,” she said.

But Mr. Cuomo’s announcement also raised a host of questions: Would the new technology work? Has it been effective elsewhere? Why did the governor wait until the last minute to do this? Transit advocates wondered how much the construction would cost and raised questions over whether Mr. Cuomo, who controls the subway, had made the decision unilaterally.

Mr. Cuomo appeared pleased to have stepped in to save the day. The decision, he said, would be a “phenomenal benefit to the people of New York City.”

Mayor Bill de Blasio’s office, however, sounded a note of caution, saying the whiplash move by the Metropolitan Transportation Authority, which oversees the subway, “was certainly no way to run a railroad.”

“So long as this new strategy proves to be real, the mayor thinks this is great news for L-train riders,” Eric Phillips, a spokesman for the mayor, said in a statement. “But like everyone else, the mayor thinks the M.T.A. has some real explaining to do about how it has handled this for the last few years.”