North of Tunkhannock (Nicholson) Viaduct on Route 11 there is its little brother, the Martins Creek Viaduct, between Hop Bottom and Kingsley.
Less dramatic height-wise is the former Pennsylvania RR bridge over the Susquehanna above Harrisburg at Rockville. (That was featured on several poster-sized PRR calendars back in the day.)
There is another DL&W viaduct over the Delaware River at Slateford, between Portland and the Delaware Water Gap. That one I've been to within the last decade.
I'm sure there are many more, although for being impressive, the Tunkhannock I believe was and still is the largest reinforced concrete railroad structure in the world. The DL&W was heavily into that in the early 1900s.
There are some major structures out in the Pittsburgh area too.
Thanks for the good info. Off topic, but I was thinking of this thread today and there's a great viaduct in Manhattan on 125th Street. Also, tons of elevated subway tracks in Queens and Brooklyn...kind of fun shooting from the platforms.
There are several long RR bridges in western PA. If you look to the west from the rt 70 bridge near Belle Vernon, you can see one that crosses the Mon (and still in use). Google's street view shows it well. There are also a few you can see from 76, going north from New Stanton. They're mostly north of the exit for 376 (which goes towards Pittsburgh).