The Cargos tab lets you name the kind of loads that your freight cars will
carry around the layout.
Cargos are important to SwitchList both to help determine where freight
cars should realistically travel to, and so that your operators can
know what their freight cars might be carrying.
To add a new cargo, press the "Add" button and provide the named fields. To remove an existing cargo, select it and press the "Remove" button.
Each cargo is associated with six pieces of information: a description that will be diplayed in SwitchList and in the documents it produces, the industry that is the source of the cargo, the industry that is the destination of the cargo, whether the cargo arrives at a fixed or random rate ("fixed rate" checkbox), what freight car type can carry the cargo, and the number of freight cars per week that should be generated. Any of these settings can be changed at any time.
Cargo sources and destinations can be any industry listed in the Industries tab. Industries can be real industries on real sidings on the layout, or can be imaginary industries somewhere off the layout. A car destined for an industry located either in a town listed as "staging" or "offline" will be assumed to reach its destination when the car reaches any staging yard.
Each cargo may only be carried by one type of freight car. If you want the same cargo to be carried by multiple cars ("both 40' and 50' boxcars"), consider creating separate cargos for each car type.
The "cars per week" setting decides how often the cargo should appear on the layout. Each session in SwitchList is assumed to be a single day of imaginary time, so on average 1/7 of the "cars per week" should be chosen as cargos for any particular day.
The "cars per week" setting is only used for probabilities; a cargo set for 10 cars per week might find 10 cars delivered over seven operating sessions, but it's also possible for 7 cars to appear over 7 operating sessions, or 12. That randomness ensures that no two operating sessions should appear the same. If SwitchList chooses a particular cargo but none of the freight cars needed are available, then the cargo is ignored.
The "fixed rate" setting allows you to indicate whether a cargo should always appear exactly as many times as you declare, or whether the cargo should appear randomly, but roughly at the specified rate.
Cargos come in two flavors. Fixed rate cargos will be generated exactly as many times as you specify. If you say that a given cargo should appear 28 times a week, then four cars a day (28/7) will carry that load (as long as enough empty cars exist.) Non-fixed-rate cargos are allowed to be more random; they'll generally appear at the rate you suggest, but more cars or less may appear depending on the luck of the draw.
Specifically, SwitchList determines the total number of non-fixed-rate cargos to generated by calculating the expected number of non-fixed-rate cargos that should appear per week. If you have three non-fixed-rate cargos that should each appear 7 times a week (1 car a day), then three cargos will be selected. Next, SwitchList randomly selects that many cargos from the list of non-fixed-rate cargos, weighting the choices by the frequency the cargo should appear. A cargo that should appear 14 times a week will be selected twice as often as a cargo appearing 7 times a week. This selection is random, and cargos are not removed from the list as selected, so the actual number of cars getting that cargo each day might be larger or smaller than what you specify.
Once the cargos have been chosen, SwitchList attempts to find freight cars to hold each car, starting with the fixed-rate cargos, and then the non-fixed-rate cargos. If no unassigned cars of the correct type are available, then the cargo will not be assigned.
In Depth: Hints on Choosing Cargos