UPSC Civil Services 2016 Prelims General Studies Paper II

Above given figures show that train 1 leaves station A on 1st day and train 2 leaves station B on 1st day. Both the trains don't reach their destination even on the 2nd day after 24 hours as the journey completes in 42 hours.
∴ Two more trains need to be introduced at both the stations i.e., station A and B. So, 4 trains are needed in order to maintain the shuttle service.