I know you have your mind made up, but I just don't get it.

Coaches go directly from the airport. Sure, they may take up to 3 hours, because of traffic into Guangzhou and the border crossing, but you don't need to go into Kowloon. Plus just checking on some schedule, CTS (Chinese Travel Service) alone has like 22 departures a day, sometimes every 30 minutes. There are one or two other companies too. And the coach brings you directly to China Hotel with immediate connection to Garden Hotel (a few bus terminates at White Swan Hotel too). Adult price is HK$250 door to door.

Now, if you take the train, you need to get to the train station. Well, that takes 45 minutes at least. Taxi fare is over HK$300. If you take Airport Express, that's HK$90, and either a taxi connection (~$40) or wait for shuttle. Only direct connection is the A21 bus (HK$33), but that's slower. Then there are only 12 train departures a day (only 3 of them the "ktt" that others are suggesting). Sure the train takes only 1:30-1:45 or so, but you need to be there slightly earlier to clear immigrations. If you miss a departure, then it's another hour or more. And the train drops you off at Guangzhou East station, so it's another subway or taxi ride to your hotel. Fare for the train is still $180-190 for standard class, or $230 if the departure is a faster Xinshisu.

The only problem with the coach is the border crossing, which can be crowded and chaotic. I'd probably avoid that during major holidays, but during other times of the year, the coach is much faster, cheaper and more convenient.

Now, if you have other reasons to go to Hong Kong - want to spend some time in the city, etc - or if you're a train fan that's fine. But if you just want to go to Guangzhou, then I just don't see why you'd want to take the train.