There is not yet a railroad museum in Portland, although some people are working on it. As you know, it takes a lot of time and money--mostly money--to set it up and keep it going.
In the Brooklyn Yards, just off SE Holgate and 17th, there is the former SP roundhouse that presently serves as a storage place for several locomotives: an Amtrak F40PH, GN (orange and green) F unit, a blue Nickle Plate Alco, the steamers GS4 and the SP&S #700 are kept inside the building, and a few other rail equipment. So far as I know, the area is not open to the public, and the UP is becoming increasingly weary of people being on or near their tracks, that may include walking to and around the roundhouse. However, you can see some of the equipment from the Holgate Street Bridge, which passes over the yard and within sight of the roundhouse. The Coast Starlight and Talgo trains (to Eugene) pass by the roundhouse as they pass through the yard. However, since it is in a yard, the yard may be full of freight cars, which may block the view of the roundhouse and its contents.
And no, it is not near Union Station. In fact, it is quite a ways from it. From Union Station, walk up 5th Ave to where you can catch a #17 "Holgate" bus going southeast. The fare is $1.30 (keep the transfer, which you can use for the return trip) and get off at 17th and Holgate.