MBTA GENERAL MANAGER Phillip Eng outlined a plan on Thursday to eliminate all 191 slow zones on the subway system by the end of 2024 and restore service to regular speeds.

The plan calls for addressing 39 of the slow zones during the remainder of this year and 152 next year. Some of the work will be done on nights and weekends, but a large chunk will involve shutting down parts of the system for anywhere from 4 to 21 days. The plan calls for 19 days of diversions this year and 188 days next year.

Most of the work will involve replacing rail, rail ties, and the rocky ballast underneath the tracks, but Eng said the T will also be repairing tunnels along the Green Line and performing station and signal repairs as well.

“This is truly a pivotal time for the T as we’re looking to restore and repair years and years of disinvesment,” the general manager said. (For the MBTA’s press release and a full listing of the planned diversions, click here.)

Eng said the goal is to restore “timely, consistent, reliable service” for existing travelers and to start the process of enticing people who have stopped riding the T back to the system.

Transit advocates have been pressing Eng for some time to outline a plan for eliminating slow zones, which have mushroomed across the system in response to aging infrastructure that makes it unsafe for trains to operate at top speed.

The T successfully addressed slow zones along the Red Line between JFK/UMass and Ashmont and on the Mattapan Line over 16 days in October, which allowed the transit authority to start planning for a more comprehensive plan.

Rep. William Straus, the House chair of the Legislature’s Transportation Committee, joined Eng at a press conference at the state Transportation Building and praised the general manager for bringing together a management team capable of getting the job done.

“We now have the management capability that wasn’t there before,” he said.

Sam Zhou, the T’s chief engineer, said the diversions planned through the end of 2024 are mapped out to minimize the impact on riders, but he said other possible steps are being explored as well. For example, he said, the T is trying to attract additional private contractors from out of state and considering contracts that incentivize firms to complete the work faster than scheduled. He said the MBTA has never done that before.

Zhou also said the T is considering shutting down one track on a section of a subway line for repairs while keeping the other track open for trains carrying passengers. Once work is completed on the one track it would be reopened and the other track would be shut down for repairs.

“We’re in flight and we have to change engines, that’s what we’re doing here,” he said.

Eng presented a slide at a subcommittee meeting of the MBTA board of directors that showed the state of the current subway system by line. The Red Line, for example, has 66 speed restrictions, its track is 31 years old on average, and its signal system is 40 years old on average. The Green Line has 69 speed restrictions and its track and signal systems are an average of 20 and 25 years old, respectively.

The Orange Line has 42 speed restrictions and its track and signal systems are both an average of 38 years old. The Blue Line has 14 speed restrictions and its track and signal systems are on average 23 and 25 years old.

MBTA board members expressed astonishment at the Red Line numbers. “That’s insane to me,” said Robert Butler of the signal system with it average age of 40 years.

Eng was less alarmed. “Given the age of our system, these numbers are not surprising to me,” he said.

Eng said track typically lasts 35 years. He also pointed out that the T faces the same challenge with the trains it is running on the Red Line, many of which are more than 40 years old.