WP 921-D was the last unit delivered in WP's first order for the F7 model, all delivered as four unit sets lettered A-D (A and D units had control cabs). These "F-units" served until newer models with higher horsepower and better visibility rendered them obsolete. The WP was slow to retire them, however. The railroad was always too short of money to completely replace many older engines, which could still earn a profit. By 1977, when most railroads had long since replaced their full carbody style locomotives, the WP still rostered four. That year, two of them, including the 921-D were sent to Morrison-Knudsen in Boise, ID for rebuilding.
WP rebuilt 917 and rewired 918 in Stockton.
For the next three years, the newly christened "Fab 4" continued to earn their keep while generating great publicity for the WP.
After the Union Pacific merger, the 921-D was retired and donated to us here at the Feather River Rail Society.