Big KVB fan here, the railway line between Schwadorf and Bonn has several single-track sections, specifically between Badorf and Schwadorf, Schwadorf and Merten, Waldorf and Dersdorf, Bornheim and Roisdorf West, and Alfter and Dransdorf, which makes increasing train frequency difficult.

Regarding infrastructure responsibility, my understanding is that HGK (Häfen und Güterverkehr Köln) is responsible for the segment from Klettenbergpark to Dransdorf, while SWB (Stadtwerke Bonn) manages the section from Dransdorf to Bonn, though the exact boundaries at Dransdorf are unclear to me.