After more than eight years of construction, Sound Transit started running trains this month into the new Northgate Station, scheduled to open in September.
The first test trips, often at low speed, are designed to check the catenary system where overhead wires transmit power to the electric trains.
The $1.9 billion, 4.3-mile extension, which includes underground Roosevelt and U District stations, is expected to serve 45,000 daily passengers.
Elected officials in 1996 declared a goal of Northgate service within a decade, but costs far exceeded their early estimates. When completed, the new tracks are expected to carry people between Northgate and downtown Seattle in 14 minutes.