what is the reason a running session is best from yard to yard?
I’m not sure it is. I could also be an out and back session starting from a staging yard. It could a passenger layout and go from station to station. If could be a coal layout going from a mine to a power plant.
Some of the latest thinking is to run strictly transfers from say BNSF to UP. Transfers aren’t run by timetable so while a lot of trains can be moved there is a more relaxed less rigid structure to the whole thing with a lot less pressure on the participants.