I have two sw engines. Each one operates on a trigger. The first goes into the yard, couples to a consist and then takes the consist to a factory and loads. It returns the cars to another spur and uncouples and returns to the stall and waits for the trigger. A portal train brings empties and leaves them on the first track and then goes to the spur and picks up the fulls and takes them to the portal. On the exit it trips the trigger and the process starts over. The second sw operates much the same with a 10 minute wait before it begins. This system works great but you have to state in the trigger rule the name of the loco. You might call the trigger train "Chem-Train" or "Milk-Train" and then that is the only loco that can trip the trigger.