On bahn.expert I found an unusual train DZ 5259 from Rüdesheim to Hanau:
This train does not appear in regular customer-oriented train search engines such as HAFAS. According to Fernbahn.de, DZ stands Zug des Sonder- und Charterverkehrs, meaning a train for extraordinary or charter traffic.
I think it could be fun to travel on such a train. How can I find them? Of course, it's not a given that it's possible at all to book travel on this particular train, but maybe on some excursion trains it is possible. Is there some way to search the timetable directly for such planned trains and find out who operates them?
In this case, it might be the Museumseisenbahn Hanau organising the train, as they report they ran a charter train between Hanau and Rüdesheim on 12 June 2024 for a company event, although they (at the time of writing) have no information on a train running 10 July 2024.
Edit: I saw the train stopping in Offenbach, where it had to wait a while to be overtaken by other trains. Talked to the driver, who confirmed it was a private event (geschlossene Gesellschaft) on a train operated by the Museumseisenbahn Hanau.