It is important that trains depart on time. If a train does not depart on the scheduled time, it may affect the other trains on the same route, and there is a chance you and others will miss your next train.
The whistle signal
Just before the scheduled departure time, the conductor gives a whistle signal. Then the doors close, and the train leaves. Make sure you board the train before the whistle signal.
What to do when it is busy
Most important is to let people who want to depart the train get off first. At the beginning and at the end of the train, there is often more space with more opportunities to sit. Make sure you are on this part of the platform before the train arrives.
On the platform screens, you can see the departure time. Make sure you are on the train before this time.