more questions about : one way ticket fly to Guangzhou...

I do have china visa , good for 10yrs. The 144hr visa free transit does not apply to me. If I take high speed train from Guangzhou to Hong Kong, I don't think I will need any visa to enter HK, a passport is good enough.
My plan is to fly into Guangzhou with one way plane ticket, then go to Hong Kong by high speed rail. To Singapore from HK, then back to US from Singapore.
Is there any other legal document required bedsides local residents ID or foreigner passport to take the high speed rails?