Again: as is always said best source for train bookings is the relevant local railway: in this case the German equiv of this forum-so many times mentioned, is drehscheibe-online.de. It is also divided up in many branches. I myself, though very often with the cheap DE-ticket travelling in DE, do not do freight, but from what I understand there is not really a national/centralised source for this, one has to check various sites of the multitude of freight-running operators, plus that actual operator may even be different from the responsible company for that ride.
Suggest you take a look there first.