Route 6, Danielson, Conn.
"New England's Most Modern Dining Car"
Opened in 1954 - Still Operating and a 3rd generation family business. Although the original address of the diner was listed as Danielson, it actually resides in Dayville. Dayville is a historical district and was added to the National Register of Historic Places in 1988.
A short history of the diner can be found on Zip's website.
I love that Bill Griffith who draws Zippy the Pinhead occasionally features this diner in his comics.