Service on part of the MBTA’s Red Line is now halted for 24 days for track work as the transportation agency promises to get trains moving at speeds they haven’t attained in decades. “I’m going to go so far as to say that what we’re doing here is historical in nature,” MBTA Chief Operating Officer Ryan Coholan said during a Thursday ride on the Red Line with NewsCenter 5. During some weekends within the work period, MBTA officials have warned the project would also impact the adjacent Commuter Rail lines. The Braintree Branch will be closed from JFK/UMass through Braintree until Sept. 29. Project plans call for replacing 18 miles of track along the stretch, removing more than 20 speed restrictions.”At the completion of the project, it’s going to set the stage for what the Red Line can look like — and what it will look like,” said Coholan. “We’ll …