A month-long service disruption began Tuesday on the Red Line’s Ashmont Branch, as shuttle buses replaced train service between JFK-UMass and Ashmont, including the Mattapan trolley line, through next Wednesday (April 9). That will be followed by three weeks of “shuttle trains” on the Ashmont branch.

The disruption will allow crews to perform track work and infrastructure upgrades, including replacing the aging “Ashmont Diamond” crossover track, which allows Red Line trains at the end of the branch to switch tracks and turn around. MBTA General Manager Phil Eng described it as “proactive” maintenance to avoid future delays.

“We want to replace it before it continues to fail,” he said last week.

So, what does that mean for your commute in the near-term?

April 1 to April 9: During weekdays, shuttle buses will run every three to five minutes on the Ashmont branch and every five to six minutes on the Mattapan trolley line. (They’ll be less frequent on the weekend.)

While the buses are free, they’re not as fast as the trains. So you’ll want to budget, to use the T’s word, “ample” extra time. Officials say riders traveling from Mattapan to Park Street should expect their commutes to take at least an extra 45 to 60 minutes.

What are other options? For the next nine days, the T is making the Fairmount commuter rail line and the 18 bus free. 

Meanwhile: The rest of the Red Line should see normal service — and there’ll even be a little more frequency between JFK/UMass and Braintree, with trains running every seven to eight minutes.

What’s a shuttle train? Since the T won’t be able to turn trains around while the crossover is being replaced, they’ll just be running a single train back and forth on each of the two tracks. On the northbound track, the train will stop at all five stations from Ashmont to JFK/UMass, then throw it into reverse. Due to technical reasons, it will run “express” on the way back and skip Fields Corner and ShAwmut. Meanwhile on the southbound track, another train will do the opposite.

Wait times will be longer — 13 minutes at Ashmont, Savin Hill, and JFK/UMass, and at least 25 minutes between trains at Shawmut and Fields Corner. The trains also won’t be free.

But the T says the shuttle trains should be faster and more accessible than buses. MBTA officials also said it’s the first time in recent memory they’ve used shuttle trains for such a long period of time — and on double tracks.