As part of the tunnel project SEPTA implemented a diametrical mode of operation. Heretofore the Pennsylvania and Reading trains had terminated in their respective terminals. Besides making transfers difficult, this led to congestion and reduced capacity. With the opening of the tunnel, Pennsylvania trains would run through the tunnel on to matched Reading lines, and vice versa. This would reduce congestion at the downtown stations, as very few trains would terminate or originate at them, and reduce the number of potential passenger transfers as each train reached more destinations. The original plan for the system was made by University of Pennsylvania professor Vukan Vuchic, based on the S-Bahn commuter rail systems in Germany. Numbers were assigned to the Pennsylvania lines in order from south (Airport) to northeast (Trenton); the Reading line matches were chosen to balance ridership, the physical characteristics of the lines, and the location of yards. An additional consideration was avoiding crossovers on the trunk lines. and to attempt to avoid trains running full on one side and then running mostly empty on the other. Vuchic recommended seven lines: