We have been on a trip to Hungary (4 days), Croatia (4 days), Serbia (2 days), and Romania (6 days). We have ridden trains. Hungary has good trains. Romania has lower end trains, but they are not expensive (Timisuara-Oradea - 24 Lei - $6).
Riding on trains has led us to have a number of conversations with locals. We simple speak English to one another. Eventually, a local will ask us where we are from. We just came to Oradea, and had a very interesting conversation with a young man the age (exactly) of our children. He is a border guard. He showed us a video of people trying to get into Romania from Serbia. He told us about his father during the Romanian anti-communist revolution (30 years ago this year) who was in the army at the time.
When arriving into Beograd, Serbia, I had a nice conversation in German with an older guy. He helped us figure out where the station was, and how to ride the bus into town.
So, if you want interactions with locals, riding trains seems to work for us.