Will be travelling next May in Germany. Purchasing the single country Twin Saverpass.
My question: Is this valid on the Gebruder Kolb ships that run up and down the Mosel River? (I know that it includes KD ships). And is it valid on all the rural buses that run between villages both along the Mosel AND in the Black Forest? I know each region has its own regional bus system, but this IS a national level pass....

Your question is best put to the experts on the passes -- Raileurope. I suggest you email them (look at the Raileurope site) and ask your specific questions for good answers. I've done this numerous times regarding specific cities and whether or not city busses or metro systems are covered. They always respond within a day or two at most, often within minutes!!!

If you buy your pass from www.ricksteves.com, they will authorize you to email 3 questions to their rail center for no charge. Usually offer something with the pass, like a book, or in my case 30% off any purchase. His website has very good information on the usage of passes (of course, not for something so specific as what you want to know).
FYI: many sources say that any International rail office will validate any pass, but I could not convince the office at Schiphol/Amsterdam to validate my German Railpass. The office in Arnhem, NL did so, however. Go figure.
We did not use the pass for any of the river cruises.

Whoa. You're saying that if you buy the pass from Rick Steves and you have more than three questions about it, they won't answer or they will charge for the questions? That's outrageous. It may be nice to give some free stuff, but I'd sure expect a place that sold me a pass to answer unlimited questions for free about its use. Am I out of line?

The railpass is only valid on the K-D line, not on the Kolb line. It is not valid to my knowledge on the regional buses, but only on the Europabus buses along the Romantic Road.

The "Regiolinien" buses ARE covered by the regional daypasses you can buy in Germany; the Rheinland-Pfalz ticket, for example, a regional daypass for the Mosel and Rhine area which costs 21 euros/day, covers regional trains and is also good on these buses. The pass is good for up to 5 people traveling together. So it might be good to opt for one of these instead of purchasing an additional railpass day (purchase at stations in Germany.) There is a Regiolinien bus line between Wittlich's train station and Bernkastel, for example. Here is a page with links to timetables for the Regiolinien:

www.spnv-nord.de/sites/flyer/start.htm

Also, the regional transit authority wherever you happen to be usually has some kind of pass available that covers both trains and the many other local buses for smaller areas than those covered by the Rheinland-Pfalz ticket. Check with local train stations or maybe tourist offices.

If the above responses hasn't answered your questions an even faster way is to call Raileurope toll free. Their number is listed on their website under the "Contact Us" option.

Patrick, I don't fully know R.S.'s setup for questions. I only availed myself of the email method, didn't need more. I do believe he runs a consultant service that charges for 30-min or 1-hr increments, more like a travel agent service (not limited to rail). So if you need more help for rail questions, I'm not sure if you'd have to fall into the consultant category.
There was no markup on the passes, ordering was easy, and the site's info was quite comprehensive.

I had the identical pass, but it was three years ago. A brochure accompanying the pass specified the validity. At that time the pass was valid on trains of the DB network and boats of the KD line only. A 75% reduction was granted on busses on the "Romantic Road" and "Castle Road." Busses in regional transport were not covered.