Don't confuse a mixed freight train with a passenger train with freight cars.
two distinct different ways of running specially with freight brakes.
Southern was still calling these passenger grains. There was no caboose. The ETT's said passsenger trains carrying piggyback cars would observe passenger speed limits, but not exceed 70 mph. In most of the territory I ran around in, that last was meaningless, as the curves kept everything below 60 for the most part. At the time, Southern was strongly promoting the big john high capacity grain hoppers, so I have a feeling that they were on the Carolina Special for some sort of special move. Again, this train was in the timetable, both public and employee as a passenger train, not a mixed.
Traveler: I don't know what the limits were for the cars used in the Warrington freight era. For the Auto Train, CSX imposes a speed limit of 70 mph on a line that is 79 mph for the other passenger trains. Basically, the limits would be the lesser of what would be regarded as safe for the car or the speed limit set by the host railroad company.