Downtown Seattle Transit Tunnel
The Downtown Seattle Transit Tunnel (DSTT), formerly also known as the Metro Bus Tunnel, is a 1.3-mile-long (2.1 km) pair of public transit tunnels in Seattle, Washington, United States. The double-track tunnel and its four stations serve Link light rail trains on the 1 Line as it travels through Downtown Seattle.
Source: Wikipedia — Downtown Seattle Transit Tunnel (CC BY-SA 4.0)