Upvote:4
I assume you are worried because you are a UK citizen and the conditions for the German Rail Pass state that this product is only available to passengers whose place of residence is outside Europe, Turkey or Russia.
This is not very clear for people who are citizens of Europe but live elsewhere. I found the following clarification in the FAQs of www.germanrailpasses.com:
Europeans with permanent residency outside Europe can travel with a German Rail Pass, provided they can present their proof of residency document in a non-European country when requested by rail staff.
So you should be fine to buy this pass and use it with your Hong Kong permanent resident card.
Side notes: