Düsseldorf-Berlin is available for 19,90€, I doubt flights are much cheaper than that. You just have to check sites like https://bahn.guru/ https://bahn.guru/

Düsseldorf-Berlin is available for 19,90€, I doubt flights are much cheaper than that. You just have to check sites like https://bahn.guru/ https://bahn.guru/ to find the cheapest connections.