In New York City, the L line currently handles about 400,000 passenger trips each weekday, with about 225,000 of those riders taking the line under the East River each day and the others traveling within Manhattan or Brooklyn.

During the repairs, the trains will continue to operate in Brooklyn between the Williamsburg and Canarsie neighborhoods, but will not run west of the Bedford Avenue station in Brooklyn. All five L train stops in Manhattan will close, along with the tunnel.

On Monday morning, New York City’s first deputy mayor, Anthony Shorris, said the city was concerned that the state-run authority had not yet provided details on alternative service plans for L train riders.

“While we recognize the need for the M.T.A. to perform these important repairs and upgrades, we are deeply concerned that it would announce an 18-month shutdown of this critical service without a clear plan or a commitment of resources for mitigating the impact of this closure on hundreds of thousands of New Yorkers,” Mr. Shorris said in a statement.

The authority is considering several travel options, including extra subway service on nearby lines and new bus or ferry service. Cars are likely to be added to trains on the G line, which is known for having short trains that riders must sometimes scramble across the platform to board.