With so many people using the train system in Japan, it's inevitable that at some point someone is going to fall of the platform and land on the train tracks. Assuming this happens there are several structures in place to keep you safe.
First is a row of steps going along the platform to help you climb back up before the train comes.
Second, if you can't make it to one of the steps, there are alcoves marked in yellow along the platform where you can get out of the way of the train.
I hope I never have to use either of these, but it's good to know they're there just in case.