I was sitting in an aisle seat on the AVE train from Cordoba to Barcelona when a suitcase from the passenger bag rack above my head belonging to another passenger fell directly onto my head. It was painful and left me with a cramped neck for a couple of days. I'm lucky that my injury wasn't much worse than that. The rack obviously isn't wide enough to balance all sized bags correctly.
If you have an aisle seat, I suggest you look carefully at the bags above your head to see if any of them look as if they might fall to avoid the same thing happening to you.