Just a couple of things... The F-2's were an intermediate between the FT and the F-3's. The Burlington Route had a few F-2A's and use them with F-3A's and B's. They weren't too common, and I can't tell you the exact differences. Probably minor internal changes. Some F-3's (notably the ones the "Q" used on the California Zephyr) did have steam boilers to heat the passenger cars, but their water tank capacity was limited. By stretching the F-7A's to accomodate larger boiler water tanks, the result was the FP-7A. They were used with F-7B's where greater pulling power was required. Same with the FP-9's. In the E-unit catagory, there were a small number of EA's, used by, some roads. Perhaps the more unique passenger units were the E-5's used exclusively by the Burlington. How were these different? They were basically E-6's, but with a slightly different engine room window location. The most recognizable difference was the fluted stainless steel side panels located below the windows. These were intended to mimic the fluted stainless steel bodies of the original Pioneer Zephyrs. The nose came to a point like the E-6's. The "Q" bought mostly A (cab) units, but also had some cabless "B" units which they m.u.'d with the A's as horsepower demands required. The reason some railroads utililized F's for passenger service had to do with traction versus speed requirements dictated by terrain. E's, with their A-1-A trucks generally rode more smoothly, especially at higher speeds.
