New Delhi, a sprawling megacity of 20 million people, is regularly ranked the world's most polluted capital city, with traffic exhaust a main driver of the toxic smog that permeates the skies, especially in winter. Delhi's patchwork public transport network struggles to cater for a booming population, with long queues snaking outside the city's underground metro stations each evening and overloaded buses inching their way down clogged roads. IMAGES AND SOUNDBITES