Hongkong to Guangzhou via High Speed Railway

Hi all,

Singaporean traveling to Hongkong and will be going for a day trip in Guangzhou. Have a few doubts in mind. Hoping someone could answer in detail.

1. How long does it take to clear the immigration from Hongkong to Guangzhou via HSR?
2. Is there any immigration counter/ custom at Guangzhou South Railway Station, if so how long does it to clear?

Thank in advance!