It's about how trains are running. Consider a rail network connected like A-B-C . With terminating trains you'd have one train run A-B-A, and another train run

It's about how trains are running. Consider a rail network connected like A-B-C . With terminating trains you'd have one train run A-B-A, and another train run C-B-C. With through trains you'd have one train run A-B-C and another train run C-B-A. Terminating trains have to stop and reverse, which takes quite a lot of time because the driver has to go to the other end of the train. Through trains can just continue in the same direction, so it is a lot faster. Because a through train occupies the track for less time you don't need as many tracks to serve the same number of trains per hour.