A dual mode bus is a bus that can run on power from two different sources, typically electricity from overhead wires and some 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. Seattle, WA until late 2004 used similar dual-mode buses in its downtown bus tunnel.