In 2004, Seattle replaced its "Transit Tunnel" fleet with diesel-electric hybrid buses, which operate similarly to hybrid cars outside the tunnel and in a low-noise, low-emissions "hush mode" (in which the diesel engine operates but does not exceed idle speed) when underground. The need to provide electric power in underground environments brings the capital and maintenance costs of such routes closer to those of light rail, and raises the question of building or eventually converting to light rail. In Seattle, the downtown transit tunnel was retrofitted for conversion to a shared hybrid-bus and light-rail facility in preparation for Seattle's Central Link Light Rail line, which opened in July 2009. In March 2019, expansion of the light rail in the tunnel moved busses back to surface streets.