It appears to me that you have received pre notification from a court of law regarding an upcoming case. This usually asks how you wish to plead, and if guilty do you want to attend court for mitigation purposes.
This generally means that it has gone beyond the train operators involvement in regards to a pre court settlement. For whatever reason they did not receive your correspondence nor change of address details. Technically, this does not matter as they can, if they wish, go straight to a prosecution. However, they generally prefer, if there are no aggravating factors, to just settle - easier all round.
You have plead guilty and asked to attend court, under normal circumstances this would be a bad move due to the additional costs this involves. However, it may just buy you time to settle with the train operator.
You must, as a matter of urgency, contact the prosecution department, both by email AND registered mail, and explain your situation. Clearly explain that you informed the prosecution department about the situation AND the future (now current) change of address. Reiterate that you wish to settle. Good luck.
If successful, ensure that the train operator withdraws the prosecution and obtain proof of this in writing. Then, if still required to attend court, you will have proof to show the court officials (hopefully a situation that doesn't arise).