Beacon Hill neighbor Roger Pence, who works with Sound Transit, said on the mailing list:
…these power lines have nothing to do with Sound Transit. We didn’t know they were going up either! This is a “betterment” project entirely the doing of Seattle City Light.
Later reports indicate that City Light upgraded the service to power trolley buses and in expectation of increased demand in Rainier Valley.
(Tangentially, I noticed the trucks on Stevens between 12th and 13th yesterday morning, and when I called City Light to inquire about them , I was told it they were performing sewer work. “TV sewer inspection.” Odd. Updating with links as they appear.)