Transit visa for Hanoi to Hong Kong by train

Dear Sir/Madam,

I wonder if you can help. My wife and I and our 2 children (Aged 15 and 13 ) would like to travel to Hong Kong by train from Hanoi in Vietnam in August 2019.
We would like to take the overnight sleeper train from Hanoi to Nanning, leaving Hanoi Gia Lam station around 21:40 and arriving Nanning main station around 10:10 next morning . We would leave
Nanning Main station on train D3625 at 13:22 and arriving Guangzhou South at 17:22. We would then change trains and travel from Guangzhou to Hong Kong, leaving Guangzhou South at 19:08 by high-speed train and arriving Hong Kong West Kowloon Terminus at 20:08.

As I understand this would take less then 24 hours and therefore I understand we should be eligible for a transit visa g. Could you confirm that this is possible and if so how and where we can obtain a transit g.

Many thanks

Kind regards

David