Just a word of caution for anyone booking their own train connection, this is usually only done if you book train and flight on the same ticket. The DB calls this Rail&Fly and essentially the train becomes a leg of the flight. So if/when the DB screws up, your "flight" is delayed and treated just like a delayed plane.
I suppose that it may depend on the airline how they handle such cases. I do not remember which I had used then, but it might have been Lufthansa.
In my case I had bought the train tickets separately from DB (online) and the flight tickets directly from the airline (also online). At the airport I have just shown the train tickets and it would have been easy to verify that indeed it had arrived with a huge delay, so it was not my fault.
https://www.bahn.de/service/informationen-buchung/bahn-flug/...