MILW Lines West Segment Reopening?

Perhaps. According to this study done by the State of Washington just released this past December, the Ellensburg - Lind segment of the main line, roughly 110 miles, could be rebuilt to relieve congestion on the BNSF main line over Stampede Pass.

Who's mainline?

I think he is referring to the old Milwaukee Road mainline.

Otto,

Indeed, this study refers to the Milwaukee Road's former Pacific Extension main line to Puget Sound (also known as part of the railroad's Lines West). For those who aren't aware the line is quite famous for its 400+ miles of main line electrified operations which lasted until 1974. Sadly and needlessly Lines West were entirely ripped up starting in 1980, some 1,000+ miles of railroad lost.

Theres not much conjestion on Stampede. Maybe Stevens. Every once in a we hear a rumor about something like that.