Just before Amtrak, the fastest stretch of trackage that diesel powered passenger trains ran on was the 100 mph trackage on the IC near Carbondale. The Milw and Santa Fe both had many 90 mph sections of trackage. These speeds were authorized by the ICC due to maintenence/signaling features on these routes. The fastest ‘start to stop’ scheduled diesel passenger run was on the CB&Q - it was listed many times in the old Trains Speed Survey.
Gear ratios in diesels are based on the ‘maximum rotational speed’ of the actual motors that are geared to the drive wheels. If they spin too fast under load, the copper windings with become soft and ‘creep’ outwards until they ‘short out’ on the motor case. This is called ‘bird caging’ or a ‘ground short’ and the motor needs to be isolated until they can change it out. In EMD E units, the maximum speed could vary between 85 mph and 117 mph, depending on the gear ratio. The CB&Q E units were geared with a 98 mph speed(very popular by the time of Amtrak). I rode in several runs between La Crosse and St Paul, and on one run(1968), we had #32 up to 106 mph going through Hager City, WI. I was suprised that there was no overspeed alarm/limiter tripping off. I am sure if there had been a heavy load on the engines, it might have ‘bird caged’ one of the traction motors. Many of the early E unit purchases had the famous 117 mph gearing. IIRC, the GN purchased pairs of E7’s with this gearing, and overheated traction motors on Marias Pass as they could not keep the train above the minimum continuous speed for the electrical gear. BTW, I think Alco adverties the PA’s with a 120 mph gearing - anyone who can verify this?
Jim