You can use any train of the participating operators.

However, you should only use the Interrail app as a last resort. It doesn't get enough updates to be reliable.

Use the operators’ own websites and apps. For international planning, the DB website or DB Navigator app are a good choice.

Thanks for that, it makes more sense. From home to London there are a lot of trains but the app only shows one option.  I’m so looking forward to a short trip and Christmas shopping…