Obviously there is the Horseshoe Curve. That I believe is $7 a person, but it is worth it. There is the Railroaders museum. I'm not sure about the cost, but they have historical equiment. Buit most could be seen from the parking lot. The Altoona Juniata shops are visible from the street. These are still active NS shops so no trespassing. Galliztan has the tunnel park, with the Allegheney tunnel. Cresson has the helper station. Both Gallitztan and Cresson have inns right along the NS mainline. Then there is the Allegheney Portage Historical site, an incline RR used to haul boats over the hill.
"Why would you take a train to go see another train?"
Some people just don't understand.