in the four intermediate stations. The work began on April 5, 1993. When the viaduct reconstruction finished on March 31, 1997, full <7> express service was reinstated. Throughout this entire period, ridership grew steadily. In spring 2018, express service west of 74th Street was suspended temporarily so the MTA could fix the supports under the center track at 61st Street. Early 21st century upgrades Automation of the line In January 2012, the MTA selected Thales for a $343 million contract to set up a communications-based train control (CBTC) system as part of the plan to automate the line. This was the second installation of CBTC, following a successful implementation on the BMT Canarsie Line. The total cost was $550 million for the signals and other trackside infrastructure, and $613.7 million for CBTC-compliant rolling stock. The safety assessment at system level was performed using the formal method Event-B. The MTA chose the Flushing Line for the next implementation of CBTC because it is also a self-contained line with no direct connections to other subway lines currently in use. Funding was allocated in the 2010–2014 capital budget for CBTC installation on the Flushing Line, with scheduled installation completion in 2016. The R188 cars were ordered so the line would have compatible rolling stock. CBTC on the line will allow the to run 7% more service, or 2 more trains per hour (tph) during peak hours (before retrofit, it ran 27 tph). However, the system had been retrofitted to operate at 33 tph even without CBTC. The first train of R188 cars began operating in passenger service on November 9, 2013. Test runs of R188s in automated mode started in late 2014. However, the CBTC retrofit date was later pushed back to 2017 or 2018 after a series of problems that workers encountered during installation, including problems with the R188s. The project also went over budget, costing $405 million for a plan originally marked at $265.6 million. The whole line was cutover to CBTC operation on November 26, 2018, with the completion of the segment from Hudson Yards to the north of Grand Central. Completely independent of the CBTC installation is the 7 Subway Extension (see below), which features both CBTC signals and fixed-block signaling. The extension will also increase line capacity. Extension westward In the 1990s, the Metropolitan Transportation Authority (MTA) began exploring the possibility of a Flushing Line extension to New