"Seděj ve špatným vlaku?"

Translation: Are they sitting in the wrong train?

[JiDvok7]

Why "the"?

[BoneheadBass]

Although the course generally does use "a" when there is no demonstrative pronoun in the sentence, I'd suggest that it wouldn't be possible to translate this without "the."

"Are they sitting in A wrong train" would be incorrect in English.

[zeibura]

The words "right" and "wrong" usually take a definite article in English - you say "are we going the wrong way?", even if there are many possible "wrong ways" one could go. Similarly "are we on the wrong train?" (the word "sitting" is entirely unnecessary)

[BjrnMrtens]

How would you say "Are they sitting in a bad train?" it Czech?

What a "bad train" means is another question, but maybe it could be rat infested, or it could be full of Manchester United supporters while the passengers in question are Manchester City supporters :)

[VladaFu]

It can be with the very same words - I would than understand the technical quality is bad. Football supporters - It is still a wrong one, is't it? Definitely špatný(ej) in Czech.

[zeibura]

In the first sense you could use "hrozný" instead of "špatný" which is a bit more specific, and stronger.