sound transit (st), officially the central puget sound regional transit authority, is a public transit agency serving the seattle metropolitan area in the u.s. state of washington. it operates light rail service (link light rail) in seattle and tacoma, regional sounder commuter rail, and sound transit express bus service, as well as managing the regional orca fare card system. in 2017, sound transit services carried a total of 47 million passengers and averaged 157,000 riders on weekdays.