South Korea -Beijing - north Korea

Will I be eligible for the 72 hour free transit if I fly from South Korea to Beijing, then travel via Train to North Korea via Dandong (2 hours in train terminal)?

I would be eligible for the 72 hour free transit if I flew from South Korea to Beijing, then flew from Beijing to North Korea. However, would I be eligible again a week later flying from North Korea to Chengdu via Beijing (staying in terminal) before flying Chengdu to Lhasa or would I need to get a Chinese entry permit as well as a Tibet Permit?