According to the Exit-Entry Administration of the Ministry of China, to secure the right of Hong Kong and Macao residents to visit China mainland, the validity period for mainland travel permits for Hong Kong and Macao residents will be extended from May 8th.

With the resumption of cross-border trains, an increasing number of Hong Kong and Macao residents have started to visit mainland China. To accommodate this demand and reduce pressure on application agencies in Hong Kong and Macao, the permits' validity period has been extended for individuals who could not obtain permits due to the pandemic, and for those with expiration dates between January 1, 2020, and December 30, 2023. The validity period is now extended to December 31, 2023. Permit holders can go through exit/entry border inspection formalities and travel to and from the mainland using their original permits. After entering the mainland, passengers can visit the nearest exit/entry administration department of public security organ at or above the county level to apply for the renewal of their home return permits.

For further information on the announcement, document renewal, and port clearance, Hong Kong and Macao residents can call the Exit and Entry Administration, PRC's 12367 service platform for detailed inquiries.

Further reading:
Bullet Train from Beijing to Shanghai: Timetable, Tickets, Prices
China Entry and Travel Restrictions in 2021
Baggage Rules of China Rail