The West side of Steel Bridge has great views of Southbound Amtrak trains leaving Portland Union Station ( PUS ). I spent an hour or more there just last month, photographing MAX light rail as well as the S/B Coast Starlight. Several Portland and TriMet transit police drove by & left me alone. The money shot here would be a train on the lower level and light rail on the upper level roadway at the same time. Note that the lower railroad level of the Steel Bridge lifts up into the upper level, without the upper level having to lift.
Also worth noting, there is a lower level walkway across Steel Bridge, on the South side, that gets you to within 10 feet of so of the RR tracks; be careful as this walkway is heavily used by bicyclists who DO NOT CARE that you are also there. This walkway connects the Willamette Riverfront walk to East Portland.