Dortmund Central Station has been closed to train traffic since early Saturday morning.
The signal box there failed because there was water ingress after a storm, said a spokesman for Deutsche Bahn.
Since around 3:30 a.m., no trains have been able to enter the station.
Long-distance traffic will be diverted.
Regional trains would also be diverted or turned back just before the main station.
There are significant delays and disruptions.
Among other things, many football fans who wanted to watch the game between BVB and Werder Bremen in the afternoon are also affected by the blocking.
How long the disruption will continue is unclear, says the railway.
Passengers should inform themselves before starting their journey on the Internet or via the DB app.
The heavy rain not only had consequences at the train station: several streets and cellars were flooded in Dortmund, and the fire brigade was deployed more than 20 times.