Arrive at CAN airport and leave by train to HK with the 24h TWOV

Yes, its is possible, but since this is a bit complicated, I suggest you to book a ferry ticket from Nansha port (reachable by metro from the airport with two changes) and from there to Hong Kong. To be allowed to do that, you will have to book your ticket online, in advance through CKS - Chu Kong Passenger transport company website. There you will book your ferry ticket for Hong Kong and you will receive an e-confirmation. You will print out the mentioned e-confirmation and you will use it as a proof for your transit and also to collect your real, paper ticket at Nansha ferry terminal.

If you insist to take a train and especially if you intend to have an overnight in Guangzhou, I will explain you how to book and how to use a train to Hong Kong.