Hopefully, this is not the problem it once was, in the days of parallel mergers and large fleets of dissimilar locomotives.
Going back to the 70's and 80's for examples: The C&NW concentrated their ALCOs on the Winona, MN-Rapid City main line. The 17-stall roundhouse at Huron, SD was the maintenance base for these units. They even brought C-628s down from the Iron Range and leased additional ALCO units (ex SP and NYC) to handle as much freight as possible behind ALCOs.
They were gone by the time of this line's sale to DM&E in 1986. Typical power on this line by then was SD-7's.
In the same time period, Milwaukee Road found itself in possession of several pools of minority brand switchers. They centered these locos around maintenance bases where parts supplies for that particular brand could be concentrated. Examples were: Fairbanks-Morse: Janesville, Madison, Milwaukee, Beloit (of course) Wisconsin. Baldwin: Winona, Red Wing, Twin Cities, MN. LaCrosse, WI. ALCO road switchers: LaCrosse, WI.
These minority units never strayed far from their maintenance bases.
EMD road power was the standard system-wide. GE road units tended to be kept on the West End, then, after the Lines West shutdown, were used in pool freight service out of Bensonville. These were high-horsepower, late model units. EMD equivalents were the major power on fast road freights.
First generation EMD units ( SD 7&9, GP 7&9, homemade GP20's and "SD 10's", F units) were used on local freights and secondary main line freights.
A special pool of mu-equipped SW 1's were based at LaCrosse for use on ex-narrow gauge branch lines with very light bridges. Former passenger FP 45's were repainted into freight colors and run in the fast freight pool out of Bensonville. Some early ALCO RS-series road switchers were chop-nosed and fitted with C-420 cabs and used on locals and work trains out of LaCrosse and Winona.