A dual mode bus is a bus that can run on power from two different sources, typically electricity from overhead lines and fossil fuel. Boston is expected to install dual mode buses on its new Silver Line sometime in 2005. Electric power is required in the exclusive right of way that runs in a tunnel under Boston harbor; Diesel power will be used to run on city streets and highways, including the Ted Williams Tunnel to Logan Airport. Seattle, Washington used similar dual-mode buses in its downtown bus tunnel until late 2004.