One of the harder parts of setting up any car movement scheme - switchlists, car cards, colored thumbtacks, or just writing up random switchlists off the top of your head - is getting the right balance of cars moving to match your imagined scheme. If you want a particular industry to have lots of cars flowing in and out, and another to be quiet, you need to make sure that your car movement scheme generates the right number of movements, and that your freight car fleet has the needed cars available.
With SwitchList, you adjust balance primarily by specifying the cargos that should be carried. Each cargo lets you specify how many loads per week should be transported on average. If you say that a cannery should receive three cars a week of empty cans, then (roughly) every other session, a carload should arrive. Because SwitchList chooses loads randomly from the cargos you provide, the car of cans might not arrive for two sessions, or two cars might arrive on the same day. This randomness ensures that your operating sessions don't become too repetitive.
When adding cargos, think about how many cars per day the industry track can hold, the ratio of incoming raw materials to outgoing finished products, and the sorts of freight cars you want visiting the industry. One quick way to look at the cargos visiting the industry is to switch to the Cargos tab and select the "Source" or "Destination" column headers to sort and group the cargos for that industry. (You can also use the Search field in the tab to search for all cargos with a specific name to see, for example, how many cars of cans are sent to other industries.)
The Cargo Report may also be useful for balancing cargos. The Cargo report lists the total number of incoming and outgoing car loads, broken down by industry and type of freight car. If you expect Cannery A to have twice as much business as cannery B, then the Cargo Report can help you see if Cannery B truly has only half the business.
The second way to affect layout balance is to make sure that you have the needed freight cars to carry all those loads. When you start a new operating session or generate additional loads to fill, the Overview Tab shows a status message at the bottom describing how many freight cars on the layout are reserved for or carrying a particular cargo, and how many loads couldn't be carried because of unavailable freight cars.
110 freight cars, 97 assigned, 8 at workbench Unavailable cars: (type/number): RS (3), XM (2), FM (1)
In the case above, SwitchList's status tells us that we have 110 freight cars defined, and 97 of those either are carrying a cargo or are routed to pick up a cargo. Of the 13 cars that don't have a cargo, eight are at the Workbench for repairs. The remaining cars do not have a cargo assigned because there are no cargos for them to carry. This could be random luck (no such cargos were randomly chosen today), the freight cars might have the wrong division (east coast boxcars for loads destined for the west coast), there may be too many freight cars for the cargos you've specified, or the freight car could be a type that only infrequently carries cargos.
The second line tells us about the opposite problem - cases where a freight car load was randomly picked, but no cars were available. In this case, there were three refrigerator car loads that couldn't be assigned today because no refrigerator cars were free. Similarly, there were boxcar (XM) and flatcar (FM) loads that couldn't be carried because of a lack of cars. This may or may not be a problem. If you want all your freight cars moving all the time, you can make the "loads per week" very high so that there's always loads to fill your freight cars. If you're trying to have the cargo "loads per week" match real expectations about how many cars should arrive and depart, then this might hint that you don't have enough freight cars for the loads you want to carry.