My first piece of advice is in the title. My GPS couldn't find it, but fortunately I'd checked on a map before setting out and whilst I was there they had a query from a "lost" visitor. When you do find it you won't be disappointed. Start with the informative film to give a better understanding of the history, allow plenty of time to walk down to the Lemon House. A good place to spend a few hours.
If you own or manage Allegheny Portage Railroad National Historic Site, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.