The MBTA Green Line shutdown has started in Boston. Here's what to know.

The MBTA Green Line shutdown began Monday morning. Parts of the line that goes through downtown Boston are out of commission for two weeks while crews replace aging infrastructure. 

Here's what to know about Green Line closures:

When is the Green Line shutdown?

The shutdown is in place from Dec. 8 to Dec. 22

Why is the Green Line shut down?

The MBTA is replacing a 130-year-old wooden "trough" that is part of the original construction in the tunnel with a more durable metal trough. Crews will also install a new Green Line Train Protection System that's designed to avoid collisions between trains and enforce speed limits, and work to make stations brighter. 

What parts of the Green Line are shut down?

There will be no Green Line train service between North Station and Babcock Street on the B Line, Kenmore on the C and D lines and Heath Street on the E line.

Alternative Green Line service

The MBTA has detailed other ways for Green Line riders to get around during the shutdown, though it says trips could be at least 20 minutes longer than usual.

There will be free shuttle buses running between Babcock Street and Back Bay on the B Branch. Riders can then take the Orange Line at Back Bay for service to North Station. The Route 57 bus will be fare-free during the shutdown.

C and D branch riders can also get a free shuttle bus between Kenmore and Back Bay. Fares will be free at all surface-level stops west of Kenmore. 

Extra Route 39 buses with free fares will be added between Heath Street and Back Bay to replace E Line service. 

The Worcester Commuter Line will be fare-free between South Station, Back Bay and Lansdowne Street during the shutdown.

• MBTA
• Boston