My main reason for going to Altoona was to see the Horseshoe Curve near there. Prior to its construction, trains trying to go from points west to points east would have to go through a railroad portage. This of course took time, so the railroad looked for alternatives. When construction of the curve was complete in 1854, their alternative was there.
Ever since then, it has served as a major piece of rail infrastructure. So much so that during World War II, a group of Nazi Sabouteurs were landed to try and blow it up. They were caught before they could do it. Currently, it serves on the Pittsburgh Line of the Norfolk-Southern Railroad. Since it is one of the main lines, it is fairly busy.