I am a holder of the UN 1951 Convention travel document issued by Australia. It allows me to travel to Germany, Hungary, Slovenia and Slovakia without needing a visit visa. I have been issued a visit visa by the UK and I would prefer to go by ICE train than travel by air. This means that I will have to change once at Brussels Central Station. Can I legally travel via train or is flying my only option?
There is no equivalent of "airside" at Brussels South station. If you cannot legally enter Belgium and France, you cannot legally travel by train from Germany to the UK. Since you can't enter The Netherlands, the Dutch Flyer ticket is not an option either. At Brussels South Station, Belgian border force will have exit checks, so at this point you will be found out if you try.