Service
An industry gets served when the local runs.
The local runs on a schedule. The crew is "bulletined" by labor agreements to run on a certain "schedule", doesn't matter whether its run as a regular train (timetable schedule) or an extra. The crew is paid to run the train every day its bulletined. We have discussed the patterns of how locals are scheduled before. If the local is scheduled to operate, it operates because its being paid.
The cars are spotted or pulled when the local gets there. If the local arrives at Delta at 4 pm and departs at 5 pm, and the industry releases a car at 7pm, its not picked up until 4 pm the next day when the local gets there. If the local runs as a tri-weekly local and only services an industry on M-W-F, if the industry releases a car at 7 pm Monday, its not pulled until 4 pm Wednesday. If its released at 7 pm on Friday, then its not pulled until 4 pm on Monday. If its a branch that only gets switched one a week, on Thursday and the car is released on Thursday at 7pm, then it sits there until next Thursday at 4 pm when the next local gets there.
Back in the day, the most expensive part of running a train was the crew cost. Extra locals were discouraged. If a local runs and the railroad sends it back out for a second trip, it gets paid a second day's wages.
Also back then "overnight" and "just in time" deliveries were not the expectation. If you read ads from back then, mail order stuff would say right in the ad to allow for delivery in 30 days. And that's part of the equation. If the industry KNOWS its switched M-W-F its going to expect it to be switched M-W-F. And the industry will load, unload, release and bill cars accordingly. Its also possible the industry will use that to its advantage, billing or releasing a car before its ready, to get the car off demurrage, knowing that by the time the railroad gets there 12-24 hours later, the car will be ready.
Aerial photos are handy. Look at the number of rail served industries and the number of cars on spot. There will be a lot more cars on spot than today, but there will be waaaaaaaaaay more spots. Historic Aerials has a 1962 photo of Oil City, PA. Between all the railroads in the city, I count about 40 cars on spot between both railroads in the whole city (excludes cars in yards or on trains). Warren, PA is similar. Disclaimer : your count may vary because its a low res image and seeing individual tracks without cars is difficult and I can't see cars inside buildings.
Also you can set whatever traffic level you want. If you have 20 spots at a station and want to work every spot 3 times a session, go for the gusto. The 20-10-5 numbers were an illustration. Some industries get switched every shift, some industries get switched once a day, some industries get switched once a week, some industries get switched once a year. Really big industries can get switched more than smaller industries. If you look at today's traffic levels, industries will tend to get switched more overall and that's because the railroads have ripped out tracks and abandoned lines to industries that didn't have traffic. For example, in the previous paragraph, when I was looking at the aerials of the general area, the 1940's and 1950's topo maps showed a NYC line in that area, so i was going to count cars on that line, but by the 1966 aerial, the earliest for that area, the NYC line had been ripped up. If a line is completely gone, that's a pretty good indicator they didn't ship much.
The reason I mentioned the 20-10-5 thing was you were concerned about the size of the locals and I was pointing out that just because your railroad SERVES 200 spots, it doesn't have to SWITCH 200 spots every session.