The example I had in mind was for an S-Bahn in Karlsruhe (operated by AVG, using the S-Bahn trademark under license, but otherwise unaffiliated with Deutsche Ba

The example I had in mind was for an S-Bahn in Karlsruhe (operated by AVG, using the S-Bahn trademark under license, but otherwise unaffiliated with Deutsche Bahn) which was considered a tram, not a regular regional train while it traveled within city limits. This meant that DB tickets weren't necessarily valid, and AVG claimed as much. I expect AVG recognizes the bw tarif under which DB sells train tickets, so this probably is no longer an issue.