2008-built system was largely unusable to the decommissioning of 3G networks. Positive Train Control was implemented on the entire system per a federal mandate, which required installation by the end of 2018 with the possibility of a two-year extension. Construction began in 2017. The final segment of the system to have Positive Train Control activated was the inner Worcester Line on August 15, 2020. Most of the southside lines already had cab signals for automatic train control (ATC) prior to PTC implementation, but the northside lines did not. Cab signals on the southside were completed in 2020; cab signals on the northside will be completed in 2023. Temporary bus replacements for several lines took place between 2017 and 2022 during PTC and ATC construction and testing. The Greenbush Line opened in 2007. The Riverside-Framingham section was sold to the MBTA in 1976 as part of their larger acquisition of PC commuter assets, but the section past Framingham remained in Conrail control. In September 2009, Conrail successor CSX Transportation and the Commonwealth finalized a agreement to purchase CSX's Framingham to Worcester tracks, as well as the Grand Junction Railroad plus lines which will be part of the South Coast Rail project, to improve service on the Framingham/Worcester Line. After several years of construction and negotiations, ownership of the line was transferred to the commonwealth on October 4, 2012, with increased service on the outer section of the line beginning several weeks later. As Big Dig mitigation, MBTA rebuilt existing stations and is adding 4 new stations along the line. The first of these, Talbot Avenue, opened on November 12, 2012, followed by Newmarket and Four Corners/Geneva on July 1, 2013. Blue Hill Avenue station was opened on February 25, 2019, after many delays. Service was extended further south to T. F. Green Airport in Warwick, Rhode Island in December 2010 and to Wickford Junction in North Kingston in April 2012. This represents the first commuter service in Rhode Island south of Providence since 1981. In 2013, the CapeFLYER service began running from South Station to Hyannis on summer weekends – the first direct service from Boston to Cape Cod since 1959. Though officially a Cape Cod Regional Transit Authority service, the CapeFLYER uses MBTA equipment. A 4-mile extension of the Fitchburg Line to Wachusett station opened on September 30, 2016. COVID-19 pandemic Weekday service was substantially cut on March 17, 2020,