Probably doing similar things to what's been done with Russia, more sanctions, more sanctions against supporting nations etc.
They're soon to test a nuclear weapon and already play fun games testing missiles and having them land just off the coast of Japan. Not going to be fun once those things are nuclear missiles and en route to Tokyo.
Their economy is already almost completely dependent on China. It's hard to sanction them more.
Sanctions against Russia aren't uniquely effective in a way that sanctions against NK aren't. It's just that the threat of having your economy look like North Korea's is pretty dire.
I don't think that you can reasonably treat Yeonmi Park as a reliable witness.
North Korea's train network is 4700km long, though it is not perfect... If you want to travel from Pyongyang to Chongjin for example, you're better off flying there
They're soon to test a nuclear weapon and already play fun games testing missiles and having them land just off the coast of Japan. Not going to be fun once those things are nuclear missiles and en route to Tokyo.