public static List <List <Wagon> > GetAllWagonsByStation(List <Station> stations) { List <List <Wagon> > wagons_by_stations = new List <List <Wagon> >(); foreach (Station station in stations) { List <Wagon> wagons = Wagon.GetWagonsByStation(station.station_id); wagons_by_stations.Add(wagons); } return(wagons_by_stations); }