As the experience on the 7 line shows, it will not be easy.

It has taken six years to install the hardware on the line. Given how long this project is taking and how long it took to upgrade the L line, transit officials in the past have said that completing the work on the whole system would take 40 to 50 years.

“It’s absolutely clear that 40 years is simply too long,” said Joseph J. Lhota, the chairman of the Metropolitan Transportation Authority. “We know that will not work. We have to do it faster, we will do it faster.”

He said officials were hoping to take advantage of new wireless technology to hasten the work, but he could not provide an updated timeline.

No subway system in the world uses a wireless signaling system and some critics say it is unwise to try to move ahead with untested technology. Still, Mr. Lhota said that officials believed they would be able to determine by winter if a wireless system was feasible.

One attribute of New York City’s subway that makes it among the world’s great public transit systems — the fact it never shuts down — is also a liability since it limits the time available for repairs or overhauls. Officials try to limit the inconvenience by doing work late at night or on the weekend.

A recent tour of the work being done on the 7 line made it clear just how labor intensive it is to install a computer-based system that requires removing and replacing miles and miles of cables.