Not sure if this is the right place but anyway................................
I have had 3 people from US this month ask about getting from HK to Pek.... best way.
Although they have not all said so one at least is visiting the Sourcing Fair down in HK and wants a cost effective way to come up to another show in Beijing. She has worked out that to fly from China HK to China Mainland is an international flight and is rather pricey.
So, two options.
1/ Take the overnight sleeper train from HK station direct to Beijing, you need to book your bed ahead of time, do it direct, do NOT use any of the website touts, they will rip you for 4 or so times the real ticket price. Also, don't get sucked into what they call a "soft sleeper" is is just as comfortable
Option 2, take the KCR