It used to be possible to purchase a train ticket after you've boarded the train. As of Sunday April 1st this is no longer possible in DB Regio Bayern trains. The conductors will still be available to check tickets and give out information, in fact, the logic behind this change is to give the staff more time to dedicate to customers.
If for you are unable to purchase a ticket due to a broken machine or no counter/machine available at your station, you will nevertheless be permitted to purchase a ticket from the conductor. But they will probably check up on the information you give to make sure you're telling the truth.
Apparently the information has been posted up around the stations, but I haven't seen it yet. I only found out because we went on a train trip a couple of weekends back and they were announcing it on the trains.
DB statement in German: DB Regio Bayern bietet mehr Service in den Zügen
