doors on each side, fold-up seating areas for wheelchairs, and two bicycle hooks above luggage storage areas. 1Line trains are typically arranged into four-car sets, but until 2021 all trains were two or three cars long. The trains have a top speed of , but typically operate at on surface sections and on elevated and tunneled sections. Link uses a form of positive train control to prevent trains from exceeding the set speed limit for a given area. Trains are supplied electricity through an overhead catenary that is energized at and converted to three-phase alternating current through on-board inverters. While other North American light rail systems use technology, Sound Transit chose to use 1,500 V DC to reduce the number of electrical substations, which are spaced approximately apart. Sound Transit placed its initial order of 31 light rail vehicles in 2003, and added four more vehicles in 2005 for the extension to SeaTac/Airport station. The cars were assembled in Everett, to comply with Buy America requirements, and delivered from 2006 to 2008. Another 27 vehicles were ordered for the University Link extension in 2009 and were delivered from 2010 to 2011. The 1Line fleet is stored and maintained at a operating base in SoDo, between SODO and Beacon Hill stations. It opened in 2007, at a cost of $74 million to construct, and has a capacity of 105 light rail vehicles, including nine bays inside the maintenance building that can hold 16 vehicles. 1Line trains are operated and maintained by King County Metro under a contract with Sound Transit that was renewed in 2019 and is set to expire at the end of 2023. In September 2016, Sound Transit approved a $554 million order to Siemens Mobility for 122 S700 "Series 2" light rail vehicles that will serve planned extensions to Northgate, Lynnwood, the Eastside, and Federal Way. Another 30 vehicles were added to the order in April 2017, bringing the total to 152 vehicles. The first Series 2 car arrived at Sound Transit's maintenance facility in June 2019, featuring the same seating capacity but a wider central walkway and other new features. The first Siemens cars entered service on May 14, 2021. A satellite maintenance facility, with a capacity of 96 vehicles, is planned to be constructed in Bellevue to accommodate part of the new fleet. Future plans Sound Transit's expansion ballot measures, passed as Sound Transit 2 in