Many railroads running FTs found a problem due to their being designed to run as A-B sets with drawbars. Each unit created 1350 horsepower, so an A-B set had 2700 HP, and two A-B sets coupled back-to-back had 5400HP. At that time, mainline freights required around 4000 HP. So the FTs had sort of a "Goldilocks" problem - one A-B set didn't have enough horsepower, and two sets (back-to-back A-B-B-A) had a lot more power than needed.
The Burlington solved the problem after WW2 by buying F2 A-units from EMD and putting them together with A-B sets of FTs. F2s had the same 1350 HP, so what the Burlington called an "FT-2" set had 4050 HP..."just right" for mainline trains.Some other railroads did something similar (C&NW for example), so seeing an A-B set of FTs with an F2, F3, or F7 on a mainline freight in the transition era was pretty common. A few bought the FTSB (FT - Short Booster) B unit, which had drawbar connections at both ends, so the railroad could have an A-B-A set of FTs (but the short booster didn't have space for steam generators for passenger service).
BTW Santa Fe did briefly have some FTs in the warbonnet scheme. After WW2, there was a backup of months or years to get new diesels, so while Santa Fe had F-units on backorder they repainted some freight FTs into the warbonnet scheme and added steam generators so they could fill in on passenger trains as needed. By about 1950 ATSF had enough new passenger F's and the FTs went back to freight service in the blue and yellow freight paint scheme.
