@jonobie In theory it’s part of city building code for places that admit the public. it is routinely ignored except by eg restaurants and bars. the tunnel transit stations should all have public toilets per this code — similar code in Auburn and other outlying suburban towns forced Sound Transit to put restrooms in at Sounder stations.
https://www.seattle.gov/documents/Departments/SDCI/Codes/SeattleBuildingCode/2018SBCChapter29.pdf