24 hour round trip from HK to Guangzhou by train

Hi, I am French and would like to use the 24-hour direct transit policy. My Chinese visa just expired and I have to be in Guangzhou tomorrow. I already have my round trip tickets for Guangzhou booked, but I wanted to know if it is a must to travel to a 3rd country after my stay in Guangzhou under this policy? Or can i still go back to HK within 24 hours from arrival in Guangzhou?

Thanks!