Horseshoe Curve, PA, well where to start. First I would recommend a visit to the Railroaders Memorial Museum in downtown Altoona, PA ( there's a discount if you purchase both a ticket to the museum and the curve on the same day ). Altoona offers a lot of photographic opportunities for the railfan, with many bridges that span the tracks ( activity along the line is high so expect a lot of freight action, plus two daily Amtrak trains ). Horseshoe Curve while not the most photogenic location still offers a lot of excellent photographs for the railfan as well. I second the recommendation of Gallitzen tunnel, great photo opportunities exist there and the town even took the time to put viewing holes in the bridge spanning the tracks ( very rainfan friendly feature in my opinion ). As a bonus Altoona, PA is approximately 1 hr driving time away from Cumberland, MD. Why is this important you ask? well the Western Maryland Scenic Railroad is located in Cumberland, MD. If you haven't been to Cumberland or taken a ride on this excellent tourist line I recommend it. They operate both steam and diesel excursion up to Frostburg, MD, with dining options and an excellent carriage museum located in Frostburg.