King County Metro fleet

Metro's first low-floor trolleybuses, New Flyer Xcelsior model XT40 vehicles, entered service in 2015.

Metro maintains a fleet of electric trolleybuses that serve 15 routes along almost 70 miles[13] of two-direction overhead wire. This is the second largest trolleybus system in the United States by ridership[14] and fleet size.[15] The trolleybuses are valued by Metro both as zero-emission vehicles,[16] and as vehicles well adapted to Seattle's hilly terrain.

Metro's trolleybus fleet consists of 174 entirely low-floor New Flyer Xcelsior coaches.[17] Of the total, 110 are 40-foot (12 m) vehicles (model XT40) and 64 are 60-foot (18 m), articulated buses (model XT60).[17] The buses include an auxiliary power unit, to allow them to operate off-wire for up to 3 miles (4.8 km).

Occasionally Metro will use diesel or diesel-electric hybrid coaches on trolley routes. Reasons for doing this include construction (weekends only),[18] overhead wire maintenance or events that require coaches to go long distances off-route, "coach changes" (replacing a bus in service that has developed a problem) or to add temporary additional capacity. The latter two cases sometimes lead to diesel buses being used, in order to get the replacement or supplementary vehicle into service as quickly as possible; diesel buses can reach the point of entry into service faster, as they do not need to follow the overhead wires when deadheading.

New Flyer DE60LF diesel-electric parallel hybrid bus operated by King County Metro

Metro operates the largest fleet of hybrid buses in the country. The first hybrid buses were purchased in 2004 for use with routes that operated in the Downtown Seattle Transit Tunnel.[19] The National Renewable Energy Laboratory conducted a one-year comparative study between conventional diesel and hybrid-powered buses operating on a typical King County drive cycle. Results showed that the hybrid powered buses lowered fuel consumption by 23%; NOx by 18%; carbon monoxide (CO) by 60%; and total hydrocarbon (THC) by 56% when compared to conventional diesel buses. Those results have led Metro to purchase hybrid buses exclusively since 2005 (with the exception of the all-electric trolley buses).[20][21] Metro now has over 700 hybrid buses in the fleet, with more on order.

Buses equipped with the GM-Allison E^P50 and the Allison H 50 EP parallel hybrid systems have a special "hush mode" that allows the buses to operate solely on electric power, reducing tailpipe emissions and noise while operating in the Downtown Seattle Transit Tunnel.[19] Before entering the tunnel, the operator pushes a button that puts the coach into hush mode. While buses are inside stations, the coaches operate solely on electric propulsion (although, while the doors are closed, the engine still rotates in order to operate auxiliary loads). In between the tunnel's stations, the bus uses electric traction to get to 15 mph (24 km/h), after which a combination of the electric and diesel motors are used. The operation of the diesel engine allows the batteries to recharge. Hush mode is normally deactivated by the operator as they exit the tunnel, but the mode will be automatically deactivated after the coach has traveled a certain distance.

Orion VII diesel-electric series hybrid bus operated by King County Metro

Metro's newest buses are equipped with the BAE Systems HybriDrive, a series hybrid system.[22] In these buses an electric motor turns the wheels, with power provided by a generator attached to a diesel engine and regenerative breaking. Any excess power is stored in batteries on the roof of the bus. Because the diesel engine is not directly propelling the bus, it can operate at a more steady, fuel-efficient speed.

Buses delivered after 2014 are equipped with the upgraded HybriDrive Series-E which uses electrically powered accessory systems (alternator, air conditioning, air compressor, cooling fans and steering pump) to increase fuel efficiency and allow the diesel engine to stop when the bus is stopped and the batteries are sufficiently charged.

Metro began testing three new Proterra Catalyst battery electric buses in 2015.[23] The coaches are capable of traveling over 26 miles[24] before the battery needs to be recharged. A special fast charge station located at the Eastgate Park and Ride allow the bus to be fully recharged in under 10 minutes, during the driver's normally scheduled layover.[25] These new vehicles get the equivalent of 20.8 MPG, which is over 6 times better than the 3.18 MPG seen on Metro's series hybrid electric coaches.[26][27] The coaches were purchased with support from a $4.7 million Federal Transit Administration grant and entered revenue service on February 17, 2016.[28] They operate on shorter routes (due to their limited range) on the eastside, specifically Routes 226 and 241.[26][29]. Metro has committed to purchasing 120 electric buses with the option to purchase up to 80 additional vehicles by 2020. In 2017 and 2018, Metro tested electric buses with ranges of 140 miles from several manufacturers[30], and plans to have a zero-emissions bus fleet by 2040.