In 1939-40, a quartet of F-units barnstormed across 35 states, logging 83,764 miles on 20 railroads and proving once and for all that diesels were the wave of the future. One of EMD's largest customers was the Pennslvania Railroad. On Pennsylvania mainlines from the east Coast to Chicago, new EMD diesels displaced the Pennsy-designed steamers that had been the symbols of the "P Company." While other diesel manufacturers competed with EMD, the various versions of the ubiquitous F unit — FT, F3, and F7 — became the icons of the diesel revolution.