When on top of the train, why is it important to focus on what’s in front of you all the time? What type of injuries does it lead?

Focusing on what's in front of you saves you from being being hurled from the train by a branch.

To avoid the branches, he sways from side to side. All of the riders sway in unison, ducking the same branches--left then right. One moment of carelessness and the branches will hurl them into the air.