Safety barriers
After a cluster of terrible, fatal accidents involving trains and commuters in the 1990s, Japan set about a program of universal station platform safety barriers - refitting existing stations and including the barriers on all new stations. Safety barriers like these are a common sight now, but it will take more time for them to be universal throughout the country. I'm shocked by the common absence of such barriers at train stations in the rest of the world.
I admit that it was kind of cool in the old days, before the common use of barriers, to stand on a platform while arriving/departing trains whooshed by mere centimeters from my face. I could stick out my finger and touch the moving train. Although that was an invitation to injury, I actually did it sometimes.