Turn left on Broadway as you exit Union Station. Follow Broadway for 1.1 miles to Market Street. Turn left and go 0.2 miles to 4th Avenue. Total distance 1.3 miles.
All this is in downtown Portland, where it is tough to go too wrong. Odd numbered streets go southbound (Broadway is where 7th would be), and Broadway has bike lanes. You climb slightly on Broadway and will descend on Market.
I needed a route from the Amtrak station to a motel up by the airport. I used Google maps, set on by bicycle. Then I used the street view version to check it out. It looked fine so I entered the route in my gps. Everything went great until I encountered a bridge under construction. I followed the detour signs to get across. My gps helped me get back on my original route.
I did check out Broadway and just wanted to get a feel for how bike friendly it is. My wife and I will riding folding bikes each carrying a large backpack (part of a month and a half rail trip).
Broadway is as good as any other urban street here (and better than 5th since it doesn't have light rail tracks and has a lane). If for some reason you want to go even easier, the Green and Yellow light rail lines (MAX) pretty much go directly along your route from Amtrak to PSU.