It is well known that the Germans are a logical lot. Unfortunately, the way in which logic is applied differs from country to country. When buying train tickets, do not expect the person standing behind the Deutsche Bahn ticket counter to make any logical jumps for you. I and others have made the same mistake.

Iain: Hello, I'd like the cheapest train ticket to ....... .

Ticket agent: Yes that is €..... .

Iain: Could you tell me when the next train is?

Ticket agent: In 20 minutes on platform 2.

This is where you have to be careful. Your question has been correctly and logically answered: The next train is in 20 minutes. You have, however, made the first fatal mistake with German-style logical thinking. You have not asked if your ticket is valid on the next train. Only after you're sitting on board are you informed by the conductor that a surcharge is required for intercity (IC) or high-speed (ICE) trains. No amount of explaining will get you out of this situation. So always make sure you ask which trains your ticket is valid on. Otherwise you'll be forced to fork out cash that you weren't planning to spend.