Another alternative is to use electric propulsion, which Seattle's Metro Bus Tunnel and Boston's Silver Line Phase II implemented. In Seattle, dual-mode (electric/diesel electric) buses manufactured by Breda were used until 2004, with the center axle driven by electric motors obtaining power from trolley wires through trolley poles in the subway, and with the rear axle driven by a conventional diesel powertrain on freeways and streets. Boston is using a similar approach, after initially using trolleybuses pending delivery of the dual-mode vehicles in 2005.