Although you can get everywhere where regional trains connect, there are some regional trains on which the Deutschland ticket is not valid. Note that those trains are a tiny minority of all regional trains. The restriction applies in particular to regional trains that are operated by DB Fernverkehr (rather than DB Regio or other companies), such as shown in this table at bahn.de, which has a footnote that could become important if you are on one of the affected trains:
Fernverkehrszüge (z.B. der Eisenbahnverkehrsunternehmen DB Fernverkehr AG oder FlixTrain/Flix SE), die für die Nutzung mit Fahrkarten des Nahverkehrs freigegeben sind, dürfen innerhalb der genannten Landestarife und Verkehrsverbünde mit einem Deutschland-Ticket nicht genutzt werden. Dies gilt unabhängig davon, unter welcher Zuggattung oder Zugbezeichnung diese verkehren (z.B. ICE, IC/EC aber auch RE). Andere Fahrkarten des ÖPNV/SPNV gelten innerhalb ihrer jeweiligen räumlichen Geltungsbereiche weiterhin.
Translation:
Long-distance trains (e.g. of the railway companies DB Fernverkehr AG or FlixTrain/Flix SE), which are approved for use with local transport tickets, may not be used with a Deutschland-Ticket within the named national tariffs and transport associations. This applies irrespective of the train category or train name (e.g. ICE, IC/EC but also RE). Other ÖPNV/SPNV tickets are still valid within their respective areas of validity.
The t-online source is from April 2023 and since then it appears the situation has changed for some lines, so you should double-check if you travel on any of those lines:
- Berlin Hbf – Elsterwerda (RE 17)
- Berlin – Eberswalde – Prenzlau (RE 28)
- Potsdam – Berlin Hbf – Cottbus (RE 56)
- Dresden Hbf – Freiberg (Sachsen) – Chemnitz Hbf (RE 17)
- Dortmund – Dillenburg (Hessen) (RE 34)
- Erfurt – Weimar – Jena – Gera
Those trains are strange beasts, as a search with bahn.de reveals:
It’s the same train, but the choice is yours if you want to pay the Intercity price or the Regional Express price.
Those trains essentially count double as intercity and regional trains. They’re intercity trains where regional tickets are generally allowed, but the Deutschlandticket is not. Fortunately, (I think) all the stations along those lines are also covered by (much more frequent) normal regional trains, so you can still reach all the relevant stations. Just be careful to not get onto one of the rare regional trains that does not allow the Deutschlandticket. For the overwhelming majority of regional trains, you are fine with the Deutschlandticket.
The Bahnhof Limburg Süd is only served by ICE trains, so you can not take a train to that station on the Deutschlandticket. It is, however, served by some local buses, so it is possible to visit the station if you are for some reason interested in it.
Honorable mention goes to the Frankfurt Flughafen Fernbahnhof, which is served almost exclusively by ICE and IC trains, but is also visited by the RB58 line and so does not qualify. (Most Deutschlandticket-eligible trains serving the airport stop at the nearby Frankfurt Flughafen Regionalbahnhof instead.)
Credit:stackoverflow.com‘