class Tren:
    def __init__(self, nom):
        self.nom = nom
        self.__vagons = []
        
    def __len__(self):
        return len(self.__vagons)

    def __str__(self):
        return "Tren {:s} de {:d} vagons".format(self.nom, len(self))

    def __getitem__(self, i):
        return self.__vagons[i]

    def afegir_vago(self, vago):
        self.__vagons.append(vago)

    def capacitat(self):
        s = 0
        for i in range(len(self)):
            s = s + self[i].capacitat()
        return s