As far as I know, Athearn GP7/9 never did go to scale width hood until the Genesis models.
For railroads in Kansas City, the West Bottoms area is really interesting, so is the Fairfax Industrial Area/Quindaro Yard. Most of this is UP but all railroads ran in the area.
If thinking about more of a switching layout rather than a roundy layout, have you considered modelling a terminal association railroad? In Wichita, the terminal association is supported by all the railroads in town and has a dedicated crew (train and rail maintenance). However, the motive power is supplied by the owning railroads depending on how much of the traffic they generated for the terminal in the previous year. If the ratio is 50% UP, 30% BN and 20% ATSF, the following year UP would provide motive power for 6 months of the year, BN power for 4 months and SF for 2 months. A lot of times the power donated to the terminal is older power which makes for interesting viewing. Much larger terminal railroads may justify their own dedicated locomotives. BTW, back in the day, when Wichita had a railroad depot downtown, it was run by the terminal association, meaning that any passenger cars being moved were moved by terminal crews.
Regards,
Nick