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As mentioned Cologne is one of the biggest bottle necks in Germany. It is run beyond capacity. There are plans to expand but the curve radius makes that difficult and DB seem reluctant to run more ICE via Köln Messe/Deutz only.
There are a number of sleeping cars certified for CD. DB City Night Line and many non Czech operators used to go there. I assume the problem is the lack of availability of extra sleeping cars. The market seems to be empty.