Well, I guess I have to second the Johnstown suggestion being a resident. Plenty of NS action on the main and if you come on a weekday you can also catch the CSX local into town and the Conemaugh and Black Lick switching in what is left of the mills.
I would also suggest Connellsville and the west slope of Sand Patch, such as Ohio Pyle or Confluence. Over the past couple of years business has picked up at Connellsville as a lot of the rack and pig trains that run over Sand Patch will swap blocks of cars there, and there is also a local that works out of there to Rockwood, serving as a feeder to the S&C sub up to Johnstown. If you are lucky you can get some W&LE and South West Pennsylvania action there as well. Also plenty of easy access through out town.
By the way you aren't missing much in Altoona these days. However if you like vacant buildings, druggies, and black Dash 9's, then that is the place for you! If you are heading that far east Cumberland is a good choice. Just as much going on, and much more accessable (there is actually occupants in the buildings downtown as well).
Greensburg and Latrobe aren't too bad either, and there is also the yard at Pitcarin on NS. On CSX West Newton is an okay spot and there are a couple of places between there and Connellsville to the east. One place that I have found quite interesting is the small town of Dawson, the tracks split the town in half in sort of some psuedo street running.
Hope this helps
Scooter