Actually I think it matters. More trains means less frequency, so the stuff has to wait less time at the station before it would be transported. As far as I know both the number of trains and the time to take the travel matters. But if you want to transport for long distances, where the travel time itself is close to 20 min, no matter how much trains you run, you will not fit into the 20 min rule. And even if you would, who wants to see coal transporting trains following each other with 2-3 wagons? I want a long slow train with dozens of full wagons.