Here is a link to the King County page about the BNSF right-of-way and a discussion of the background of the deal. Here is the 2004 report of the eastside corridor advisory committee. Here are the links King County included in their newsletter when the deal was anounced in 2006.
Here is the article reporting the initial deal approved by the Port Commission.
Here is the final article describing the 2008 plan for the Port to buy the corridor.
Here are subsequent articles describing how that deal came apart and the arrangements in 2009 for various jurisdictions to buy smaller segments.
Here’s the 2013 draft recommendations from the advisory committee