Flight USA to HK, then train to Guangzhou, then train back to HK, then fly back to USA.

Looking to get clarification on whether this trip would fall under the 72 hour visa free stay:

USA to HK (flight) -then- HK to Guangzhou (train) -then- Guangzhou to HK (train) -then- HK to USA (flight). Total trip is 3 days.

Not sure about taking the train / sky limo from HK to GUangzhou, any help would be appreciated.
Thanks!