First, the current Chicago to St. Louis route was GM&O, previously Alton Route and never had a speed limit faster than 79 mph. However, I have heard that the GM&O was known to allow some fudging on speed limits where conditions permitted.*the 79mph speed limit. I've heard that Illinois Central passenger trains often operated around 100mph.
Second, there was a section of the ICRR in Illinois that had a passenger train speed limit of 100 mph. I don't know the limits of this section otherwise than "in Illinois". Much of the remaining ICRR mainline did have a 79 mph limit, and if the train was on time it was generally observed. If the train was late, at least if the premier trains, which were the Panama Limited, City of New Orleans, and City of Miami, where the engineer felt reasonable to do so, he would likely go faster. Remember these were the days of relatively short crew districts and men at the top of the seniority roster, so they knew their track well and had no intention of doing anything stupid. A random thought here, I recall in one discussion at the time, the average age of these guys on top passenger trains was around 70.