This document is an interim technical report on a comparison and evaluation of new diesel and diesel hybrid-electric articulated buses operated as part of the King County Metro Transit (KC Metro) fleet in Seattle, Washington. This interim report covers the first six months of a planned 12-month evaluation at KC Metro. The evaluation is being completed through the U.S. Department of Energy's (DOE) Advanced Vehicle Testing Activity (AVTA) by the National Renewable Energy Laboratory (NREL). Publication of the final report covering all 12 months of the evaluation is planned for fall 2006. KC Metro operates bus-only transit service in the 2,134-square-mile Seattle/King County, Washington, area. Its fleet consists of 1,400 standard and articulated buses, trolley buses, and streetcars. KC Metro recently began using 235 new hybrid-electric articulated buses that replace an older fleet of 236 dual-mode Breda articulated buses. The Breda buses were specially developed for use in the 1.3-mile Seattle downtown transit tunnel since its opening in 1990. This tunnel was built with light rail in mind but had been used only by the Breda dual-mode buses until the hybrid buses were introduced. The Breda dual-mode buses allowed zero-emission, fully electric propulsion operation inside the tunnel and diesel engine propulsion outside the tunnel or when the buses were not connected to the electric catenary system. Both the diesel and diesel hybrid-electric articulated buses at KC Metro were manufactured by New Flyer Industries. The hybrid buses include the General Motors (GM) Allison E(sup P)50 System parallel hybrid propulsion system. This fleet is the largest application of the GM Allison hybrid propulsion system to date.