Translation:In the beginning she was taking the train.
There are not doubts about the meaning of the Italian sentence: "All'inizio lei prendeva il treno". This doesn't refer a continuous action, but a repeated action in the past. The indicator is "all'inizio" + imperfect tense, that means "more then once from the beginning". Not interpretations from the context. That only means "In the beginning she used to travel by train". If that had been a continuous action, e.g. interrupted by another action, you would have said "All'inizio lei stava prendendo il treno, quando... (In the beginning she was taking the train when... ect.)". (Italian speaker)
"Would take" is marked wrong.
It's true that officially "would" implies future perfect, but it's commonly used to describe continuous past actions. "In the past she would take the train" means she was taking the train in modern English, not that she kept planning to take a train in the future. It should have been accepted.
There are obviously many ways to describe a beginning in English. Perhaps DL can not include them all. So sometimes we have to learn what DL accept snd stay with that. The important thing to understand here is that she once upon a time had a habit of going by train. Thats why it is imperfetto. It is not about one journey that started with train and continued by bus. That would have been passato prossimo. Corretto?
“Prendere” means to get or catch (also in the sense of public transportation) or to take into one’s possession. “Portare” means to carry something from point A to point B. So with a train, “prendere” is what a person does (catch the train), and “portare” is what the train does (carry you to your destination).
I understand the Italian phrase in the sense of "in the beginning, i.e. in the past, she would (regularly) take the train" (implying that at a later period she preferred the plane or the car or whatever) and consequently translated "first she used to take the train". This is rejected, however, probably because "first" is not good enough. So I think it´s rather a problem of (my) English than of Italian?