Whay they said!
In addition, you want to take only trains marked ICE or IC. The ICE is the high--speed train like the TGV in France (just not as safe). The IC is the regular speed Intercity train. You can check how much faster the ICE is on your route than the IC, and if it's worth the extra money for you. My guess is that a one-way FRA-DUS is around 60 Euros on ICE.
You should not take RB or IR trains. Those are really slow.