The Phoenix Union Station is currently owned as a storage for a certain company; However, they're looking into selling it. It could get re-acquired by Amtrak again with it being sold again. Considering the city is starting to densify and commuter rail is in the making, new platforms would have to be done underground to reduce space and grade separate it from the busy streets. Tempe's new station* is already being constructed with space underground for future station plans and on top will be the preserved original station and new mixed-use buildings. Queen Creek has defined the spot for the station when it gets built although plans are foggy.I like this idea, but I'm afraid it won't happen for a long time. Boise is neither on a densely populated corridor nor is it part of Amtrak's expansion plans. New service to the beautiful Boise union station would be nice indeed.
For the Phoenix service, chances seem to be better for various reasons being mentioned. In addition to Phoenix, relatively large suburban communities like Tempe and Mesa could see passenger railway again. As mentioned by others, I think makes sense to start with a shorter distance, between Phoenix and Tucson, if possible.
Does somebody know who possesses Phoenix union station? I would use it again as a the major train station. It is centrally located, around a bustling downtown. A new station building wouldn't have to be built, this saves holy tax dollars . A station in the suburban nowhere should be avoided, it should be in proper downtown Phoenix.
So in general, re-acquiring the station is possible and would be great. Just costs regarding new platforms and remodeling the areas within and without the station are still in the way and since we'd need to tunnel for new platforms, it would be just as expensive or only slightly less expensive than building a brand new station. New pathways and other walkable areas would also have to be made as the station does not have any direct transit connections until the commuter rail gets built. The closest line would be the South Central Light Rail scheduled to open in 2023 along 1st and Central Avenues (outside of downtown, it'll run on just Central Avenue) which is looking to be a 4-6 minute out of station connection from Union.
An idea would be to shift Union 2 blocks east onto the parking lots between 1st Avenue and 2nd Street. It would still need to be underground but gives it direct access to the South Central line, and actually makes the station a 3-minute walk from the existing Valley Metro LRT Line, along with CityScape where mixed-use buildings and hotels are. In the same distance, you can even walk to the Footprint Center. This would give it much higher ridership as when the commuter rail gets built you can easily get to and from downtown, and people from the suburbs and even Tucson not having to take their cars before and after events in Footprint Center furthers this. The ease of access simply by shifting it just a little is big, and although unlikely and expensive, this would be very beneficial to downtown!
*EDIT: The station by the way is in the middle of Downtown Tempe