So what is the relationship between the frequency and the 20 minute limit? I guess that it is adding half of the frequency to the predicted time (because that is the average time the cargo has to wait until it is picked up?)
So for example if I have a train which is transporting stuff from A to B, and that takes 13 mins, and the frequency of that line is 10 mins, then that means that the predicted travel time will be 13+(10/2)=18? So 13 min traveling, and an average of 5 min until the stuff is picked up by the train?