US to JAPAN to Guangdong, visit the Guangzhou area and confirmed high speed train ticket to HK

Sam, your itinerary is definitely and undeniably eligible for 144 hour visa free transit valid for the entire Guangdong province and you do not need any kind of visa.

It is important to understand that you will have to buy your train ticket from Guangzhou or Shenzhen to Hong Kong West Kowloon station in advance, even before you depart toward China. You will print out the train ticket confirmation and have that printout with you at hand as a proof of your transit intention. Also, you will really have to take exactly that train you have bought your ticket for. Have a nice journey.