Red Line (MBTA)

Signalling System
The line used trip-stop wayside signaling for the Ashmont and Harvard branches until the mid-1980s, while the Braintree Branch was one of the earliest examples of Automatic Train Control (ATC). The Alewife Branch was built with ATC, at which point the remainder of the line was upgraded to ATC as well. The line was under local control at towers until 1985, when an electromechanical panel was completed at the 45 High Street control room. This was replaced in the late 1990s with a software-controlled Automatic Train Supervision, using a product by Union Switch & Signal, subcontracted to Syseca Inc. (now ARINC), at a new control room at 45 High Street. Subsequent revisions to the system were made internally at the MBTA.

The shortest scheduled headway run on the line was most likely the $1 3/4$ minute interval in the schedule published in 1928. Ridership peaked around 1947, when passenger counters logged over 850 people per four-car train during peak periods. The newer ATC signaling was designed to higher safety standards, but the block layout in the downtown area reduced the capacity by 50% over the previous wayside signaling system. The net loss of capacity measured in cars per hour has not been rectified, although at the same time the platforms were lengthened to run six-car trains, which are now operated on a longer headway. The obsolete "fixed block signaling system" installed along sections of the Red Line maintains fixed distance separations between trains based on worst-case (highest speed) assumptions. When trains run at less than full speed for any reason (such as crowded conditions, track maintenance, or weather conditions), the effective carrying capacity of passengers per hour is markedly reduced. Delays in one part of the system may be propagated throughout the entire line, especially during rush hours.

During snowstorms, the MBTA runs an empty train during non-service hours to keep the tracks and third rail clear. The Red Line experienced major service disruptions in the winter of 2014–15 due to frozen-over third rails, leaving unpowered trains stranded between stations with passengers on board.