Following that link, I get to a page which does not confirm that the gift cards can be used for train tickets. If you ask the Rufus chatbot on that page, it says "No, the Uber Gift Card can only be used for Uber rides and Uber Eats food delivery services within the UK. It cannot be used for train tickets or other transport services."
I tried to check the T&Cs, but the Amazon site gives this link:
uber.com/legal/gift, which gives the T&Cs for the USA and various other countries, not including the UK.
I appreciate people may be buying these gift cards and using them successfully for train tickets despite the above, but unless I'm missing something, there would appear to be no contractual right to do so?