Several times, I've flown to West Coast endpoints and taken LD trains back to Chicago the same day. For some reason, it just feels more relaxing to me coming home by train than by airplane.
My strategy is to book an early morning flight out of Chicago so that if there are any weather or equipment delays, I still have a good chance of reaching the West Coast with ample time to get to the train station for the trip home that same day. I've done this on the Empire Builder, Southwest Chief and Texas Eagle . . .
Empire Builder eastbound - leaves Seattle 4:40 p.m.
Quick taxi or easy public transportation from SEA-TAC to King Street Station
(I haven't done a similar trip from the Portland segment, but its 4:45 p.m. departure would be similar)
Southwest Chief eastbound - leaves Los Angeles 6:15 p.m.
Quick and easy Flyaway bus from Los Angeles International Airport to Los Angeles Union Station
Texas Eagle eastbound - leaves Los Angeles 10 p.m.
Quick and easy Flyaway bus from Los Angeles International Airport to Los Angeles Union Station
I have not done this with the California Zephyr . . .
California Zephyr eastbound - leaves Emeryville 9:10 a.m.
I've never been confident I could arrive early enough in San Francisco or Oakland to catch a taxi or public transportation to Emeryville. I'd also be worried about delays. I'd be more comfortable staying overnight in Emeryville to do this one-way trip.
I've been lucky and arrived early enough in Seattle or Los Angeles that I could check my bags and meet friends for lunch or take a quick tour of the city.
Hope this helps!