Each time we go back to Altoona to spend a little time in the mountains and to watch the heavy NS action, Cassandra Crossing is always one of the highlights of the trip. You can really get "up close & personal" there as the trains pass mere feet below you.
The story is that at one time, the main road into town crossed over the railroad where the overpass is located. When the new road bypassed town, that bridge was closed. But there were miners who lived in town who complained about having to cross the busy railroad tracks and climb the hill on either side of the tracks. So a narrower, pedestrian footbridge was put in for them to cross to & from work. By the time the mines closed down, the bridge was a popular train watching spot and was left in place.
While I'm not sure of the exact distance, I'd guess you can see trains coming for about four miles in one direction and perhaps half a mile on the other side of the bridge. And the location above them gives one a very good photographic vantage point.
Should you ever get over to the Altoona area, Cassandra is about half an hour from downtown. Take Rte. 22 west towards Pittsburgh, get off at the Rte. 53 exit and go towards Lilly. Cassandra is easy to miss but is about 2-3 miles west of Lilly. The only drawback is that there is NOTHING in town at all... no place to eat, get gas... even to take a "convenience break." So if you do go there, be prepared to go a few miles in either direction for some of life's necessities. But while you are there... enjoy one of the best train watching places I know of!