“Historically, this kind of testing has proven to be the most painstaking and time-consuming for these projects, and the project’s working to see if they can get ahead of that at this point,” Mr. Haggas said.

The testing schedule for the project had accelerated in recent weeks, and that pace must continue and required “unprecedented” resources, Mr. Haggas said.

Planning for the Second Avenue subway line has progressed in fits and starts for nearly a century. The first phase of the project is set to open as an extension of the Q line, with three new stations at 72nd, 86th and 96th Streets on the Upper East Side of Manhattan. Future plans would expand the line to 125th Street in East Harlem.

Mr. Haggas has repeatedly questioned whether those three stations would open on time. On Monday, officials at the authority said that they were still working to install escalators and elevators at the stations at 72nd and 86th Streets.

At the 86th Street station, the contractor is working round the clock to finish the escalators at an entrance to meet the December deadline, officials said. At the 72nd Street station, delays in installing elevators could postpone the opening. The 96th Street station appeared to be on schedule to open by the end of the year.