pyongyang to beijing

I suspect this may not be okay, but others may be able to advise better. Only Beijing West railway station is a valid port of entry by train for 144 hours transit without visa. If your train does not arrive there then you either need a Chinese visa or you must fly to Beijing.

It's technically possible, however you may face extensive questioning at the Chinese-North Korean border, and you will be stuck in North Korea if the Chinese border guards refuse to grant you an entry permit. Also your transit time (from scheduled arrival time to the Chinese-North Korean border to scheduled departure time of your flight) must be within 24 hours. Just get a visa.