Go back Bejing (HK transit)

I want to go to Beijing, but stop in HongKong first. Do I need to apply something like entry permit to HK or I could stay in HK for 7 days without entry permit.

Can I buy a ticket to HK from U.S directly, then take train from HK to bejing?

BTW, I am a Chinese citizen (with U.S GC).