Hours? Even back with locomotive-pulled trains where you actually had to shunt the cars to a different platform that should be below 15 minutes, and today, it a

Hours? Even back with locomotive-pulled trains where you actually had to shunt the cars to a different platform that should be below 15 minutes, and today, it adds less than three minutes to the train that is longer at the platform (Ohlsdorf, Hamburg rapid transit). Provided that both trains arrive on time. And doing in in flight is much harder (and not legally possible), but if you would want to join/separate in the outskirts to get one into town and the other on the bypass, just the stopping and reaccelerating will cost some minutes.