I am using Duolingo to learn French at the moment and sometimes it asks me to translate an English phrase into French with multiple options, one using "en train" and one not (often I'm marked as incorrect because I didn't select both options). I don't know how to tell if the "en train" phrase is correct or not. I'm afraid I can't give an example because I don't understand the structure. I have tried using Google Translate in the past to work out the difference but "en train" simply disappears when translated into English.

Would someone please provide an example English sentence that can be translated both ways and explain the difference?

I am a beginning French learner so if you answer in French I won't be able to understand you! English only please!