The derailment was caused by a broken axle, which had been made brittle by sparks from a faulty grounding component on a motor. Full service resumed on September 25, 2019. 2020s On September 28, 2021, another derailment at South Station (subway) was caused because of a southbound train was traveling at a slow speed when it came in contact with the edge of the platform, causing the second car of the six-car train to derail. Winter issues and resiliency work During the unusually brutal winter of 2014–15, the almost entire MBTA rail system was shut down on several occasions by heavy snowfalls. The aboveground sections of the Orange and Red lines were particularly vulnerable due to their exposed third rail power feed, which iced over during storms. If a single train were stopped due to power loss, other trains behind it soon had to stop as well; without continually running trains pushing snow off the rails, the lines would become quickly blocked by heavy snowfalls. (Because the Blue Line was built with overhead catenary on its surface section due to its exposure to corrosive salt air, it was not as easily disabled by the icing conditions.) During 2015, the MBTA implemented its $83.7 million Winter Resiliency Program, much of which focused on preventing similar vulnerabilities with the Orange and Red lines. The section of the Braintree branch between JFK/UMass and Wollaston had old infrastructure and was largely built on an embankment, rendering it more vulnerable. New third rail with heaters and a different metal composition to reduce wear was installed, along with snow fences and switch heaters. The work required bustitution of the line from JFK/UMass to North Quincy on many weeknights. This program did not include work south of Wollaston. In July 2016, the MBTA Fiscal and Management Control Board approved a $18.5 million contract to complete work along the remainder of the southern branches. The project included all remaining third rail replacement, track work between Fields Corner and Savin Hill, signal system work between North Quincy and Braintree, and track replacement at Quincy Center, Quincy Adams, and Braintree. The work was completed in the second half of 2016. Operations and signaling , both branches operate on 9-minute headways during weekday peak hours (with a combined 4.5-minute headway between Alewife and JFK/UMass) and 10 to 13 minute headways at other times. Saturday service has 14-minute headways, while Sunday service