booking tickets for trains why long transfer gaps

It would be safer if you leave about 2 hours. Even through you are at the same station, you still need to check out the station first and then recheckin for the next trip. This process may take sometime. Also if you need to collect the ticket between the two trips, it need more time. Sometimes you need to line up for 30min for the tickets. If you do not need to collect the ticket and take the D or G types for your first trip, you can cut this time into 90 to 60min since D and G types usually runs in time and will not delayed.