The town of Grand Forks on the layout is based on the real Grand Forks located in southwest British Columbia. In its railroad heyday, it was served by three railroads, the Canadian Pacific, the Great Northern and the Spokane & BC, which is the main focus of the layout.
The industries that are modelled on the layout are a mix of prototype ones and some “could have been” fictitious ones that never actually had rail service. The big question then is which railroad or railroads should switch the industries. There are multiple possibilities: Option 1, Each railroad switches all of them; Option 2, Each railroad only switches some of them exclusively; and Option 3, Only one railroad switches all of them.
In all cases cars would get interchanged between all three railroads at Grand Forks as necessary.
So, how to decide on which option to use? Option 1 provides the most “play value” for the operators, as the industries get switched by every train passing through town, however it is not very prototypical for most industries, especially small ones, as they tended to be located on only one railroad and served by that one exclusively. Some large plants had service from multiple railroads but they were the exception.
Options 2 and 3 are both very prototypical, depending on the siting of the industries, so either would be acceptable, so we need to consider other factors to decide in the case of Grand Forks.
On the layout as it currently stands, the towns of Grand Forks and Carson have trains from both railroads pass through them, while the towns of Curlew and Darestof see only S&BC trains. In order to balance the work load between crews, it would make sense to have the S&BC trains focus on Curlew and Darestof, leaving the CPR to service all of Grand Forks and Carson. This is Option 3. Later on, as more experience is gained, some of the Grand Forks industries could easily be changed to be serviced by just the S&BC trains.