As I noted in the thread on the F7, the F9 is very close to the F7 on the outside. The most significant difference is 1750 hp for the F9 verses 1500 for the F7. The only spotting difference that I can see between a F7 and a F9 is that the F7 has 4 grilles on the a unit between the protholes on the side of the unit. The F9 has an additional grille between the ladder into the cab and the first porthole on the a units making 5 grilles total on each side. Like the F7 there was also an Fp version of the F9 which had a 4 feet longer car body to mount a steam generator in the back of the a unit. Also like on the Fp7 the truck spacing was strewtched and the extra space between the trucks allowed the mouting of a water tank for the steam generator.