Python - classes - Train, Wagon

class Wagon():

    def __init__(self, mass=0):
        self.mass = mass

    def __add__(self, other):
        tr = Train()
        tr += self
        tr += other
        return tr

    def __str__(self):
        return str(self.mass)

    def __repr__(self):
        return str(self.mass)

class Train():

    def __init__(self):
        self.wagons = []
        self.mass = 0

    def __add__(self, other):
        self.wagons.append(other)
        self.mass += other.mass
        return self

# ----------------------------------

train1 = Wagon(1) + Wagon(2)

train1 += Wagon(3)

print(train1.mass) # 6
print(train1.wagons) # [1,2,3]

train2 = Wagon(21) + Wagon(23) + Wagon(33)

print(train2.mass) # 77
print(train2.wagons) # [21, 23, 33]