>>> from trens import Tren
>>> t1 = Tren('Talgo')
>>> len(t1)
0
>>> str(t1)
'Tren Talgo de 0 vagons'
>>> from vagons import Vago
>>> v1 = Vago('WX-23', 30, 68)
>>> t1.afegir_vago(v1)
>>> v2 = Vago('YX-90', 40, 12)
>>> t1.afegir_vago(v2)
>>> len(t1)
2
>>> str(t1)
'Tren Talgo de 2 vagons'
>>> t1.capacitat()
150
>>> t1[0].id
'WX-23'
>>> t1[0].capacitat()
98
>>> t1[1].id
'YX-90'