I suppose a beautiful underpass for Fanno Creek Trail Park at Hall Blvd is just a crazy dream, but it doesn't hurt to take a look at some magnificent examples of how it has been done by other inspired people.
Note the type of arch and span of each Central Park bridge, rather than the rich materials and that they're designed to carry people rather than 4 lanes of motor vehicle traffic. The bridge shown at top, rises in the center, which would help deal with the low clearance issue at the Fanno Creek/Hall Blvd location.
Concrete would probably be the material a bridge/underpass for Fanno Creek Trail would be made of. I don't have a picture at present, but an overpass design based on something like those for Hwy 217 crossing Canyon and Beav/Hillsdale Hwy, is one I wonder if might be a possibility.
With cars running through them like mad, and being obliged to walk on a narrow sidewalk located at their extreme edges, the 217 overpasses are miserably bleak to walk under. If you block out the noise and visual clutter when walking under them, it's possible to see their design and finish, though very industrial compared to the Central Park bridges, is quite nice.