Ghost Town Trail is an impressive rail trail 40-50 miles long, consisting of multiple sections in two counties. The question is, can you ride horses on it??
Both Cambria and Indiana counties got back to me and told me horseback riding is not allowed on their rail trails. I suggest you therefore take horses and money you spend to other counties where you are welcomed!
For example the 17 mile Lower Trail in Blair and Huntingdon counties would love to have you visit. Check out my Horseback Riding Guide: Lower Trail. Also see my more general guide Horseback Riding Guide: Pennsylvania Rail Trails.
Ironically, a trail-user survey conducted in 2009 by Rails to Trails and posted on Cambria county’s website says this:

So apparently at some point they decided they couldn’t be bothered with horseback users, so screw them! Who wants to go to Cambria or Indiana counties, anyway? Buncha dickheads! 🙂