Transitting through China by train - HK to Vietnam

The train trip from HK would be as follows and is about 18 hours in China. Am I correct - the 24 hour visa rule applies? (I hold a UK passport)
Depart HK 07.25. Arrive Guongzhou Dong 09.24
Depart Guongzhou Nan 11.25. Arrive Nanning 15.26
Depart Nanning 18.10. Arrive Hanoi 05.20 (next day).
Thank you!