can you arriive in Guangzhou airport and travel to Hong Kong by train with the 24 hours visa ?

Part one:

Yes, you are allowed to arrive in Guangzhou and then proceed by train to Hong Kong.

As far as your second question is concerned, you are not allowed to take a bus from Guangzhou to Macau. The only way to reach Macau is to take a ferry from Shenzhen Shekou terminal to Macau using advance booking through TurboJet ferry website.

As far as your first question is concerned, you can do what you have mentioned, but there is one detail that can be problematic. Here are the steps you have to follow:

- Even before your departure to Guangzhou, you will have to book your train ticket online. You are allowed to book only a through train departing from Guangzhou East and arriving at Hong Kong Hung Hom railway station.
- For train booking you cannot use original MTR website because they do not provide an e-confirmations with the name on it. Instead, you have to use one of the agencies, like Travel China Guide (exactly this website) where you can book this train online.
- When booking a train, you will have to supply an address in Guangzhou where your real paper ticket will be delivered. This is the detail, I have mentioned to be problematic. If you are using 24 hour visa free transit, you probably do not intend to make an overnight in Guangzhou, so make a booking for a hotel would be a waste of money. Anyway, if you really plan to make an overnight, then this is fine for you.
- When making a booking, you will have to do it at least 7 days before the train departure and you will also have to pay a fee above the standard price to cover the expenses of booking and the ticket delivery to your accommodation.

Part two:

- When the booking is done, you will receive an e-confirmation that you will print out and bring that printout with you. This will be your proof that you are really in a transit.
- You have to take exactly that train you have bought your ticket for.

Beside the train, you are perfectly allowed to take a ferry from Nansha port, south from Guangzhou that is reachable by metro.

If you prefer this option, you will book a ferry from Nansha to Hong Kong (be careful not to book a ticket to Hong Kong airport) through CKS - Chu Kong Passenger transport website. Here, you will receive an e-confirmation after a booking, but you don't have a hassle about the accommodation and similar. You will also print out this e-confirmation and bring it with you, but you will collect your real, paper ticket right at Nansha ferry terminal.

These are the options you can use for your transfer to Hong Kong (and Macau) and you will decide which one will be the most convenient for your needs.