Hi Sofia,

I think the difference as I understand it, is that L'imparfait is used to express habitual past actions or background information to a story to "set the scene" as it were.

Typical English words used would be "was, were, would (as in  - we would do something in the past habitually)". Example.   When we were young we would visit grandma every Sunday.

In the case of "en train de faire" the sense is of "while in the process of doing something", another action happened. Example.  I was in the process of preparing a meal when the doorbell rang.

Hope this helps.

Jim