Full-speed service resumed this week, but T officials don’t have a timeline for wrapping up an investigation into why Green Line Extension tracks became so narrow that they had to slow trains down to 3 miles per hour.

A day after the MBTA announced it had lifted slow zones on more than a mile of its newest tracks, the agency’s top safety official told overseers work is still taking place to figure out the source of the baffling problem. The slow speeds began in mid-September and the narrow tracks news broke on Sept. 26.

“That is still under investigation and once that investigation is complete, we will provide a report on that,” MBTA Chief Safety Officer Tim Lesniak said at a subcommittee meeting.

Asked by MBTA Board of Directors member Tom McGee for a timeline, Lesniak replied, “It’s an ongoing investigation. We’re still trying to collect data, so we don’t necessarily have a full timeline put together yet.”

The T suddenly imposed dramatic speed restrictions on parts of the Green Line Extension’s Union Square and Medford branches last month. Officials say they were responding to scans of the tracks that determined the rails were too close together for safe full-speed operations.

Crews worked in recent weeks to widen the tracks in affected areas by pushing one rail further away from the other rail, the T said.

A few 3 mph slow zones on the Green Line Extension first appeared in the MBTA’s public speed restrictions dashboard on Sept. 13, and additional areas were added in subsequent days. The problem burst into the spotlight nearly two weeks later, on Sept. 26, when the Boston Globe reported that T officials blamed the walking speeds on tracks that were too narrow.

It’s not clear when officials began their investigation into the underlying cause of the narrow tracks.

(Copyright (c) 2023 State House News Service.

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