Just returned from the MN to DC road trip. Summit Diner in Somerset was consistently mentioned (along with Tony Packo's in Toledo) as the place to eat along the stretch from Chicago to Maryland.
Anyway...it's a great, basic old school diner. If you're making the trek across I70/76 in western PA, stop in at Summit. Very close to the exit, and solid breakfast. I enjoyed the scrapple and creamed chipped beef on toast. Great relief from a wasteland of fast food.