MBTA GENERAL MANAGER Phillip Eng on Monday touted progress in addressing slow zones on the subway system, but he declined to explain how the problem surfaced or provide a timeline of when it will be fully addressed.

In a generally upbeat presentation to an oversight hearing of the Legislature’s Transportation Committee, Eng reported “making progress, good progress” in addressing slow zones that currently cover a fifth of the subway system.

Three months ago on March 9, Jeffrey Gonneville, then the acting general manager, ordered a systemwide slowdown of the subway system because of an inability to verify that track repairs prompted by scans and inspections were being done. The T hired an outside engineer to track down what happened, and since then T officials have remained mum.

At Monday’s oversight hearing, Sen. John Keenan of Quincy pressed Eng for details, saying the speed restrictions “pretty much came out of nowhere.”

Eng, who said transparent communication is a top priority, refused to comment. “I don’t want to speculate on what may or may not have happened,” he said. He also provided no information on when track conditions will significantly improve.

Joining Eng at the hearing were Transportation Secretary Gina Fiandaca and acting chief of staff Katie Choe. Highlights of their testimony included:

Eng reported that a major contributor to the T’s hiring woes is an attrition rate currently averaging 13 percent. He called that level higher than normal.

After a visit to the CRRC facility in Springfield, Eng said he is now confident that the company can produce four completed Orange or Red Line cars a month that exceed contractual operating requirements. Keenan, the Quincy senator, said at that pace it would take five years to produce the remaining Red Line cars the T is owed – way beyond the last scheduled delivery date of September 2026.

The House chair of the committee, Rep. William Straus of Mattapoisett, is considering legislation that would change the state safety oversight agency for the MBTA, which is currently the Department of Public Utilities. Straus twice asked Fiandaca her thoughts on the matter, and both times she refused to take a position. “This is the structure that we have,” she said.