The Dowtown Seattle Transit Tunnel (DSTT) is used by light rail and buses alike and this station is beneath Westlake Mall and Nordstroms. At this particular station about 10,000 people per week board the trains and buses running in both directions.
The small image below, from the DSTT website, shows both types of transportation in the tunnel at the same time but on separate tracks. As far as I know, there is only one other multi-use transit tunnel in the U.S. It’s too bad the Alaska Way Viaduct (AWV) replacement tunnel project isn’t going as well as this one did. The AWV project is a total cock-up on the order of Boston’s Big Dig project.