Ethan - Boy, could we spend a lot of time dissecting this one! Do you have a particular division/subdivision and/or era in mind? If you're looking at the mid-to-late 1920s, a good source is the collection of old Frisco Employee's Magazines that are available in pdf format on the Springfield-Greene County Library website. Under the "Fuel Performance Records" feature every month, they would typically list a few trains for each division including train number and locomotive, along with crew and fuel consumed. I've managed to come up with a pretty good spreadsheet of locomotive assignements for the River Division during this era. It was pretty obvious when the 4000-series Mikes debuted on the River Division: all of a sudden, they were supplanting 4-6-0s and 2-8-0s.
