Hong Kong- Guangzhou (2 days)- Back to Hong Kong- US passport needs visa?

I am leaving the US and my boyfriend and I hold a US passport. We are landing in Hong Kong staying one day and going to Guangzhou- BY TRAIN, for only 48 hours, back to Hong Kong for a day or 2 and back to the US.
I have read so much about 72 free visa entry, airport, train etc... I am CONFUSED.
Do we need a visa or not?
I appreciate any input,
Thank you