If you have questions about the conjugation of вагончик or Russian nouns in general, you can practice and get feedback from a professional tutor.
Перестроенный амбар или вагончик.
A small remodeled barn, or a carriage house.
Похоже, на железнодорожный вагончик, в котором ездила Агата Кристи
It looks like the sort of railway carriage that Agatha Christie went about in.