I believe the state studied Seattle - Spokane service over the Stampede Pass route in the 1990s. I don't know if it's been seriously looked at more recently than that.
It is included in the 2010 study to revive the North Coast Hiawatha, CHI-Seattle, or the Sacajawea as I prefer to call it..
That study needs more study. It exclaimed over the scenery crossing the Cascades Range. Then it offered a proposed timetable which put the mountain crossing times after sunset and before sunrise for half the year or more. Nice departures and arrivals in Seattle, tho.
Srsly, both splits of the Empire Builder get a lot of riders Spokane-Seattle/Portland despite the ungodly hours of the Spokane arrivals and departures. And there's a lot of on-offs in Pasco, a fair-sized city about midway on the Spokane-Portland segment of the Builder.
WSDOT is aware of the potential, but it is a highway department after all. Reportedly, and unsurprisingly, WSDOT is looking forward to smoking a cigarette and taking a nap after it finishes the current $1 Billion round of upgrades Seattle-Portland.
When it wakes from the coming nap, WSDOT's likely priorities are
1) more time-shaving, frequency-adding, OTP-improving upgrades to the main trunk line Seattle-Portland,
2) upgrades up to the Canadian border Seattle-Edmunds-Everett-Mt Vernon-Bellingham-(Vancouver, B.C.),
and
3) Spokane-Pasco-Yakima-Seattle.
We'll see how the priorities sort out in a year or so, after the Cascades frequencies go from 4 trains to 6 a day (each way). Nothing succeeds like success, so if the upgraded Cascades route is breaking ridership forecasts, events (and politicians) could force WSDOT to do more trains sooner than it now plans.