They say a fire started in the kitchen area inside Jimmy John's restaurant on Saturday morning.
Employees tried to put it out but were unsuccessful and called 911.
The fire broke out on the day the Chadds Ford restaurant was to celebrate its 70th anniversary.
Investigators say nothing about the fire appears to be suspicious.
Jimmy John's started out as a roadside shack, known for its hot dogs and the toy trains in the dining room.
No one was injured but, when the fire was finally out, firefighters helped Jimmy John's employees try to salvage trains, signs, photos and other memorabilia, including a burned picture of the original Jimmy John's, taken in the 1940s.
In addition to saving keepsakes from the restaurant, Jimmy John's employees could also be seen retrieving money and coins from blackened cash registers hauled out of the building.
How soon owner Roger Stewart will be able to rebuild is unclear.
