Say you want to go from Moscow in Russia SSR to Kiev in Ukraine SSR, did you have to go through a border stop or can you freely cross through the border like in the United States or EU?
There was no inner border control in USSR since there were no marked borders. I remember traveling to Azerbaijan, Armenia, Georgia, Estonia and Latvia. I was born in 1976. We travelled by trains or by car (VAZ21011).
But there were the cities with strategic production (closed cities) which had certain limitations to get in or out.