 Commuter rail service to Snohomish County on the Sounder North Line began in December 2003 with a single round @-@ trip connecting Everett and Edmonds to King Street Station in Seattle during rush hour . Service was expanded to a second round @-@ trip in June 2005 and a third round @-@ trip in September 2007 , while an infill station opened at Mukilteo in May 2008 , also bringing additional service in the form of a fourth round @-@ trip the following September . 
