Here's a little piece of the downtown Portland streetcar map:
The main Powell's "City of Books" is on Burnside between 10th and 11th, which happens to be between the northbound and southbound streetcar lines (right where the "NS" and "CL" route markers are on this map). There's also a separate Powell's branch, which specializes in technical/transportation/engineering books, at 10th and Couch (next street north of Burnside -- the east-west streets in that part of Portland go in alphabetical order).
From my perspective, it is walkable from Union Station, but then, I'm (relatively) young.
As the_traveler noted, you can also walk from Union Station to MAX light rail (the thin green/yellow lines), take that a couple of stops, and walk along Burnside to Powell's; or you can walk from Union Station to the streetcar (the thick green/blue lines) and take that a couple of stops.
However, I saw in another thread that you're connecting from the Coast Starlight to the Empire Builder. I would not
recommend trying to get to Powell's, either walking or by transit, during that layover.