HKG to Guangzhuo by train for a 72 hours transit visa

Actually, you cannot take a train. This is not an eligible option. To be eligible for 144 hour visa free transit (it used to be 72, but it is 144 for several years) you will have to fly from Hong Kong to Guangzhou. So, your itinerary, but done by plane:

Hong Kong - Guangzhou - Taipei

Will be perfectly eligible for 144 hour visa free transit valid for entire Guangdong province.

You need a PCR or antigen test not older than 48 hours at the time of boarding your China bound plane.

Haroon, also this is not a visa, but visa free transit, so there is no visa of any kind at all.