After arrival at HK airport, do I need to wait for PCR result where I am tested, before heading to West Kowloon railway station?

You don’t need to wait at the PCR testing site. It’s actually a bad idea staying there, since the site may be crowded.
You can of course take public transportation to the high speed railway station. There’s no restriction imposed to inbound tourists. Please also be reminded that MTR (the underground railway system in Hong Kong) doesn’t accept credit cards.
You don’t need a physical result in order to enter mainland China. The government site says: “Electronic reports issued by testing institutions or the electronic record downloaded from www.evt.gov.hk can be used for cross-boundary travel. There is no need for persons travelling to the Mainland to collect paper form reports at CTC/CTS.”

Useful links:

tonychow1997 31

Source: stackoverflow.com