Supposedly, Santa Fe once had a timetable run in Kansas and Colorado of 108mph between 2 stations, but most start-to-stop times in Steffee's speed surveys were at most in the 80s or less. Several San Joaquin Valley segments regularly placed fairly high in his lists (70s and 80s) until the late 1950s. Of course, a station-to-station schedule of even mid-70s requires higher speed (perhaps much higher on occasion, especially for short segments) during "cruise" to make the schedule.

I've seen statements that the nationwide 79mph limit (absent cab signals and ATS) didn't happen until 1950.