Does baby crying train his lung capacity?

Not really. Baby's first cry is essential for a baby's heart. However, any further crying is a baby's way of attracting attention - when he is hungry, uncomfortable, or just needs a cuddle. Research suggests that letting the baby cry without intervening may lead to the baby developing an inability to cope up with stress as he grows up. So no, crying is not training his lung capacity. However, ignoring a crying baby can have adverse effects.