I've found plenty of people on forums that claim timetables in this period would only permit them to go 15-20mph, but I can't find any concrete sources that state this -that's what I'm after!
I'd like to know this info specifically for the Great Locomotive Chase. I'd like to know what speeds the chase actually occured with the "hilly landscape of northern Georgia" - unfortunately, the wikipedia page for this event has no sources or references for this section
The locomotives used in this incident were wood fired 4-4-0 "American" type. On straight and level track, they could go up to sixty miles per hour. Going up grade, or around curves would limit their speeds. Track conditions were the real limiting factor for wood fired steam locomotives. However, the Western and Atlantic railroad, where this chase took place, was some of the finest railroad track in the South, during the Civil War.
"The Steam Locomotive" by Jim Boyd