So, my trip to Germany is nearing, and I will soon have more time to pin down exact day trips from bases in Goslar (7 nights) and Bacharach (4 nights).
Two persons. Have travelled often in Switzerland and a several prior to Germany as well. Have never used the individual Laenderpaesse.
My question is this:
Several of our day trips will cross over into the adjoining state to reach our target town. Ex: Goslar (Niedersachsen) to Wernigerode (Sachsen-Anhalt). Or Frankfurt Flughafen (Hesse) to Goslar (Niedersachsen). Could we still purchase the relevant state pass and just pay the difference from the border town to final destination to the conductor on board? Or would it be an ineligible trip for the Landpass? I know there are other options like the Schoenes Wochenende ticket (and another).
For those in the know, please advise!

Yes, you can use the Länderticket and then pay the local fare for the uncovered segment. Or you can buy the "Day ticket for Germany" (Quer durchs Land Ticket) or Happy Weekend ticket to cover the whole trip. For the 2-ticket option: Whether the second ticket will be available on the train while you are using the first ticket is an open question. You should buy both before stepping on the first train if possible.

https://www.bahn.de/p/view/angebot/r..._tickets.shtml

For Goslar - Wernigerode you don't need to bother about Ländertickets.
It's €45.20 for 2 people return, fully flexible. Buy from the machine at Goslar station.
Take any train you want on that day in either direction.

You cannot buy tickets on (most) regional trains from a conductor because there won't be one.
Travelling without a valid ticket would be considered fare evasion, and you'd pay a penalty fare of - I think - €60 per person.
You can buy tickets on long distance trains, but I think they again raised the extra fee for doing so.
You should avoid when you can. Each station has a ticket machine, or you download Bahn app on your phone and buy your tickets with the phone as you go.

Frankfurt Airport to Goslar is better done with a ticket that also covers highspeed trains between Frankfurt and (probably) Göttingen.
Check bahn.com

The penalty fare was recently raised to 80 Euro per person.

And be sure to board only regional trains or commuter trains - not long-distance higher speed trains. Wonder what value regional pass has on boat trips like those on the Rhine from Bacharach?

Ok to all. Good to know the final segment (if I go that way, after further price comparisons) must also be purchased BEFORE boarding.
And yes, the fast train would be preferred from Frankfurt Flughafen to first home base of Goslar!
Thanks for responding.

2 train stations at airport - you want the closest to arrivals - the old station -labeled Lokalbahn-Bhf or Regionalbahn-Bhf validate if it says to any ticket or pass ahead of time (ask when buying).