Traaaains!!
Quicky: getting on the rail platform is easy. You show your JR pass to the employee in the booth and you’re granted passage. When getting off the platform, don’t just follow the rest of the travelers because you will get caught in the gates. Instead present your JR pass again to the employee in the booth and you’re good to go.
Fun fact: I’m getting more and more convinced that Japanese really like doing things backwards. Instead of the gates opening when you “check in” with some ticket (except the JR pass), they close when you do not. (i.e. by default they are open, instead of closed, like in the Netherlands)