train_wagon

class Wagon:
    pass

class Train:
    def __init__(self, wagons=None):
        if wagons:
            self.wagons = wagons
        else:
            self.wagons = []
    def add_wagon(self, wagon):
        self.wagons.append(wagon)

w1 = Wagon()
train = Train()
train.add_wagon(w1)