Generally a railroad only scheduled a freight train on their timetable if it was a "hot shot" train, running freight at something approaching passenger train speeds between two cities. It's possible the number of cars could vary from day to day.
If there wasn't enough freight to run the train daily (or how often it was scheduled) it would eventually be dropped from the schedule.
That was not generally true in the 50's through 70's when I was working on the railroads. On double track and CTC, where most of those "hot shots" ran, all freight trains were run as extras and moved on signal indications. Very few train orders were used.
On the Milwaukee Road, where I worked mid 60's, there were no schedules for other than passenger trains on double track lines. That included the main lines from Chicago to St. Paul and Council Bluffs, including the line through Marion. One of the hottest freight trains on the Milwaukee was the transcontinental train 261 and 262. That was the frieght train scheduile book number across the railroad, but they did not have a schedule in the operating timetable. The 1968 Aberdeen division timetable between St. Paul and Miles Cityshowed no schedules other than passenger trains 15 and 16 as far as Aberdeen.
On secondary lines, schedules for second class (usually through freight) trains might not tell you much about when trains actually ran. They simply gave a train running rights for 12 hours after the time in the schedule.
A train running at "something approaching passenger train speeds" was rare until we saw container trains. On the NYC main lines, the normal passenger speed was 80 and it was 60 for freights. The long distance freight trains with time sensitive traffic would be powered to do 60. Less time sensitive trains would have power for 40 to 50. But we couldn't have a lot of difference in train speeds, because speed differences limit train frequency and use up a lot of line capacity.
Many of the local trains that went on duty at the same time every day had schedules so that they didn't need train orders every day.