The first freight train cannot arrive at Chicago and be clear of the main track without delay to the passenger train. If the first freight train remains at Detroit, the coal train must leave Detroit ahead of the passenger train. The second freight train has taken the siding at Eugene, unable to go to Detroit without delay to the passenger train. The passenger train must move through the siding at Detroit (20 mph as opposed to 80 mph) then follow the coal train (40 mph as opposed to 80 mph) to the next available siding beyond Eugene.