During joint bus–rail operations, two types of vehicles were used in the Downtown Seattle Transit Tunnel: Sound Transit's Kinkisharyo-Mitsui light rail vehicles and King County Metro's New Flyer diesel-electric hybrid buses. The New Flyer buses, dubbed "tunnel buses" by King County Metro, were ordered in 2004 to replace a fleet of Breda dual-mode electric trolleybuses whose overhead wire was to be removed in the tunnel's renovation for light rail; 59 of the dual-mode Breda coaches were converted into fully electric trolleybuses between 2004 and 2007 and moved to surface routes, where they continued to operate for more than a decade before being fully replaced in 2016. The New Flyer low-floor, articulated buses feature a "hush mode" that allowed buses to operate solely on stored electric power within the tunnel, minimizing emissions and noise.