When the door closing alarm sounds, if not yet on the train, you should immediately accelerate and attempt to jump through the closing doors. Approximately 50% of the time, you will make it through the doors and stand looking appropriately smug inside the train.

The other 50% of the time, the doors will close too quickly and clamp around your head/arm/leg/bag. When this occurs, you should attempt to free the trapped head/arm/leg/bag. Other passengers may come to your assistance and attempt to pull apart the doors.

Inevitably, none of the attempts to free your head/arm/leg/bag will be successful and the driver will begrudgingly have to re-open the doors.

At this point, when the door closing alarm sounds again, somebody else will repeat the above.