Shipments Versus Car Roles

The original design of my CFS program used the notion of a Shipment, which implemented the basic idea of a shipper sending a load of something to a receiver, much as one would expect. The starting and ending tracks could be either in the modelled network (on the layout) or at virtual tracks in the unmodelled space. With this approach, only the loaded segment of a cars journey is defined, regardless of if it is heading in to or out of the modelled layout. As part of each car’s assignment, there is always at least one segment that is done empty, either to supply a car to a shipper, or to return a car back home. With the Shipment having only information for the loaded segment, the other segments must be generated in some way. This has led to some unfortunate side effects in terms of being able to shape car movements in desirable ways.

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Still Too Much Congestion

While running simulations of the revised operating scheme whereby we have no dedicated switcher at Grand Forks, but simply let the way freights switch the industries and exchange cars between the railroads, once again there is a pile up of cars that get “stuck” in the yard at Grand Forks. This is fundamentally no different than when we had the dedicated switcher, just to a different degree. It occurs on fewer tracks, and involves less cars, but it is still an undesirable outcome that will lead to unsatisfactory operations.

One thought was that we should try to minimize the shipments that are selected during restaging to ones that have a more clear path from staging to the first delivery point. This is called “congestion” and is a measure of how full the tracks are along the car’s route to its first destination. The restaging logic now considers only those shipments where the path congestion is below a certain level, with the idea that this will help the overall car flow by not overloading the paths that already have lots of cars along them.

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