The MBTA plans to fully close the Ashmont branch of the Red Line and the Mattapan trolley line during the second half of October. The shutdown will last between Sat., Oct. 14, and Sun., Oct. 29, to allow workers to undertake what the agency says is “critical” replacement work on the tracks, which are some of the oldest in the system.

The Red Line in particular has been plagued by “slow zones,” with long wait times at stations and trains inching along as cars and commuter rail trains fly past.

Track issues and rider time aren’t the only safety concerns along the line. The stations in the Dorchester section of the Red Line –JFK/UMass, Savin Hill, Fields Corner, Shawmut, and Ashmont – also have crumbling staircases. The Mattapan trolley extension runs from Ashmont to Mattapan Square, with stops in Lower Mills and Milton.

Officials have said that without the shutdown in October, the work would take six months to complete over nights and weekends.

Free shuttle bus service will be available for customers during the shutdown, according to the T, operating every five to six minutes during peak weekday hours, and every ten to fifteen minutes during off-peak weekdays and weekend hours, making that service faster than the current Red Line trains.

Four “open houses” will take place in September to give the public details of the construction work and allow them to voice their thoughts. As of Tuesday, the MBTA had not released dates for the events.

“Safety of the MBTA system is paramount, and this 16-day closure allows us to address many of the Red Line’s worst speed restrictions much faster than we’ve been able to accomplish during night and weekend work,” Phillip Eng, the MBTA’s general manager, said in a statement. “We understand service changes can be frustrating, and I want to thank the public for their patience while we perform this critical and targeted work between JFK/UMass and Ashmont Stations and on the Mattapan Line.”

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The Ashmont branch accommodates some 40,000 riders on a daily basis. The Mattapan line serves 3,700 riders daily.

The shutdown announcement came at a meeting of the MBTA board last Thursday (Aug. 24), with Mayor Wu in attendance. She noted that during last year’s Orange Line shutdown the agency essentially made free commuter rail available, and ridership boomed, and voiced a hope that free commuter rail would be on the table for the Red Line shutdown, too.

JFK/UMass has three commuter lines heading to the South Shore every day, including the Middleborough/Lakeville, Kingston, and Greenbush (Scituate) lines. The Fairmount Line, which leaves from South Station and runs down to Hyde Park’s Readville area, has multiple stops in Dorchester and Mattapan.

Congresswoman Ayanna Pressley, a Hyde Park resident who represents Dorchester and Mattapan and had previously lived in apartments across from Ashmont Station, also voiced support for free commuter rail during the shutdown in October.

“Riders have unfortunately been put in a situation where you’re deciding between safety and service,” she said, “and that’s happened because of decades of underinvestment and mismanagement.”

But she also said she was encouraged by the changes in leadership ushered in by Gov. Healey’s administration and she expressed hope that the embattled MBTA was starting to round a corner. “They’ve inherited a lot from the prior administration in this regard,” said Pressley, who has called for more federal funding for public transit. “This is deeply frustrating. Certainly, this burden will be felt the most by the most marginalized.”

The MBTA appeared to balk when asked about free commuter rail. “Because the Ashmont branch stations do not align with the commuter rail, there will not be a change in the commuter rail fare structure,” said an agency spokesman, Joe Pesaturo. “If customers in the Ashmont branch corridor take buses to the Fairmount Commuter Rail Line, they will pay a fare of only $2.40 (same as a regular subway fare). The bus shuttle will be free and service will be frequent.”

T officials said that during the shutdown, they plan to take advantage of “these full-access closures by identifying additional work opportunities along both lines to improve the rider experience through station enhancements, such as painting, power washing, and repairing lighting fixtures; vegetation removal; the elimination of tripping hazards; and accessibility improvements.”

Some of the stations are in sore need of such fixes and upgrades. Savin Hill was briefly shut down during a June weekend due its decrepit staircases, and various entries and exits at JFK/UMass Station, a key transit hub, have been shuttered in the last few years.

As a prelude to the half-month shutdown in October, the MBTA plans a weekend-long closure at the end of September. Shuttle bus service is scheduled to replace Red Line trains between Broadway Station in South Boston and Ashmont Station, as well as between Broadway and North Quincy, for the entirety of the Sept. 30 weekend.

“This service change allows MBTA crews to perform critical work on the Savin Hill Avenue and Dorchester Avenue bridges, to perform necessary tunnel inspections on the Ashmont Branch, and to accomplish station-brightening work at JFK/UMass Station,” the T said in a notification to customers.

This post was updated with additional information.