Kandy-Ella train safety - reading about people falling off leaning out for photos?

Not zero but not high. Doors are usually open (culture), and there's no barrier stopping people from hanging out. Falls do happen - Google will show you a few tragic stories per year, usually tourists doing extreme leans for photos on the curved sections. Practical: sitting near an open door and leaning out for photos is fine and safe. Standing IN the doorway with the train swaying at speed is not. Don't put yourself in a position where a sudden lurch = you outside the train. And no selfie stick leaning out at Nine Arch curves - that's how the accidents happen.