India's state-owned railway operates 7,000 passenger trains and carries some 13 million passengers daily. Photographer Ronny Sen has been taking pictures of passengers who travel in the crowded second class carriages. "One can safely predict a traumatic journey," says Sen.

The Indian railway network is one of the largest in the world but passengers have to wait in long queues at railway stations and rush to grab a seat when a train arrives.

The poorest travel in second class carriages which offer cheap travel without reservations. Sen shot the pictures between 2009 and 2012 - he calls the collection "Don't Breathe" to convey the claustrophobia and discomfort of the journey.

Only 72 people can be seated in these carriages and most seats are wooden benches. But Sen found more than 500 people crammed in these coaches.

These fortunate passengers, Sen says, have won the most coveted place in the unreserved coach: the luggage rack on top of the passenger seats. "They are the few lucky ones who can try to sleep in a coach where there's usually only sitting and standing space."

"Nights bring a peace that is hard to find in these noisy carriages," says Sen. "Sleep binds people together transcending barriers of caste, creed and religion."

"At night the sleeping passengers resemble dead bodies in a war zone," says Sen. "They sleep as they are, lying down, sitting and even standing."

Long journeys can be harrowing in second class carriages. Facilities have not been upgraded for ages.

A little girl travels in a makeshift hammock made out of a shawl as her mother sleeps beneath to protect her.