You write: a "$6 billion...hit to the tax plan would deliver a serious blow to Sound Transit’s ability to deliver light rail on schedule to Ballard, West Seattle, Everett.." et al. 
With $60 billion in local tax revenue forecast as coming in over the course of the next quarter century -- plus fare revenue, Federal grants (post-Trump), and new revenue that could be coaxed in the late 2020s from our train-loving population -- characterization of an alleged ten percent revenue cut as a "serious blow to Sound Transit's ability" should now be subject to serious debate. 
I know Sound Transit to be able to manage a 10% cut. (I used to work in the District of Columbia budget office hearing many tales of agency woe.)  
ST has managed excesses in cost way bigger than 10% and schedule overruns of more than a decade. Be assured it will survive punishment it deserves from October's mailing to every household in the taxing district falsely stating car tabs would be a percentage of "vehicle value."