Dept. of Transportation
Metro Transit Division

King Street Center
201 S Jackson St
Seattle, WA 98104
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Downtown Seattle Transit Tunnel and Changing Bus Technology

Photo of bus traveling through tunnel, 1990.

The Downtown Seattle Transit Tunnel (DSST) was opened in September 1990 to provide an exclusive transit right-of-way through the heart of Seattle's central business district. This 1.3-mile tunnel has 5 stations. The tunnel was conceived as a bus tunnel, but adaptable to include rail at some point in the future. Given the state of diesel bus emission control technology in the mid-80s, it was not deemed practicable to sufficiently ventilate the tunnel to allow diesel bus operation. The solution decided upon was the dual power bus. Transit history dating back to the 1930s has examples of buses with gasoline or diesel engines plus electric propulsion, so Metro was reasonably certain that this technology could be updated to serve in the DSST. Two dual power buses were demonstrated by Metro in the early 1980s. A bus procurement process yielded a bid from Breda Costruzioni Ferroviarie of Pistoia, Italy. Metro ordered 236 articulated dual power buses from Breda, and three prototypes were delivered in 1988. These buses featured a Detroit Diesel engine and a ZF automatic transmission driving the third axle, and a Westinghouse ac electric propulsion driving the second axle.

photo of Breda bus in tunnel

Operation of the prototypes in Seattle/King County uncovered various problems, not unexpected on a unique design. Metro and Breda persevered, and there were sufficient buses available to open the tunnel in September 1990 for the first few routes. The buses operated mostly as diesel buses, changing to electric only for the trip through the tunnel. Trolley bus overhead was installed in the tunnel, and remote pole raising and lowering equipment on the bus allowed drivers to change from diesel to electric and back to diesel without leaving the drivers seat (usually). Metro itself designed and implemented many improvements to this fleet, and it served until the fleet of New Flyer hybrid artics arrived in 2004 to replace the Breda dual power buses. The last revenue trip with a Breda operating as a dual power bus occurred on January 24, 2005.

photo of NF hybrid artic in tunnel

Between 1990, when the Breda dual power buses went into service, and 2004, when the New Flyer hybrid buses began operating, dramatic gains were made in reducing diesel emissions. Part of this was due to improvements in diesel engine design and electronic management, and part was due to the use of particulate traps in the exhaust stream. Use of the latest design particulate traps was not possible until ultra low sulfur diesel fuel became available. The net result of this reduction in emissions, plus the innovative use of the hybrid drive in hush mode, described below, allows hybrid buses to transit the DSST without violating any air quality standards and with relative quiet.

Allison, New Flyer, and Metro developed hush mode to minimize emissions and noise in the DSST. It operates as follows. Some distance from the end of the tunnel the system is put into a pre-charge mode if it is necessary to bring the batteries up to a certain level of charge. Some engine power is diverted into battery charging if pre-charge is necessary. At the end of the tunnel, the driver selects hush mode on the shift selector. An "H" shows up. This reduces engine horsepower. When the bus speed drops to 15 mph, engine fuel is cut off. With important accessories (power steering, air compressor, and alternator) driven by the engine, however, the engine must still be rotated to operate them. The diesel engine is rotated, or motored, by the electric motors in the hybrid drive unit. When the diesel is motored, it still makes some noise because engine compression has not been released. When the bus stops at a station and the doors are opened, the diesel is not motored and everything stops. When the doors are closed, motoring begins and continues until the bus reaches 15 mph. The engine is then fueled and operated at reduced power. This sequence happens at every station. At the end of the tunnel, the bus changes out of hush mode automatically.

The DSST was closed in September 2005 for modifications. During the closure new, insulated rails were laid, the roadway level in the station was lowered, the trolley bus overhead was removed and light rail electrification installed, and numerous upgrades to fire, life, and safety systems accomplished. All of this work looks toward the start of combined LINK light rail and bus service in 2009. The DSST was reopened to hybrid buses in September 2007.

Updated: Dec. 14, 2007