By Nandita Bose

MUMBAI, July 23 (Reuters) - They are the arteries that keep Mumbai's economy ticking, rattling six million people a day to offices, shops and factories. But arriving safe and sound for work after a trip on Mumbai's clogged railways is no mean feat.

On average 4,000 people die a year on Mumbai's railways, crushed under trains, electrocuted by overhead power lines or killed as they lean from jam-packed carriages to gasp for air. It is perhaps the world's deadliest commute.

"Everyday it's a nightmare. The train is so crowded that one has to look up at the ceiling to breathe," said Natasha Pillai, a young student in India's financial capital.

At peak hours more than 550 people cram into a carriage built for 200. Passengers fall to their deaths from moving trains or tumble under the wheels from crowded platforms.

People hang from coaches, balance precariously on the roofs of carriages or risk a ride on the bumpers between cars as they trundle along three lines and through nearly 120 stations.

The crush to board is so bad commuters take trains in the wrong direction so they can grab seats when they turn around.

Fatal accidents are so common that stations stock sheets to cover corpses, and officials spend hours entering details of mangled remains to try to identify victims in a gruesome database.

In the first four months of this year, more than 1,200 people have been killed, according to official figures. Many get run over crossing the tracks, too rushed or tired to use pedestrian overpasses.

"People here believe in shortcuts which is a very bad habit," rail official Bhagwat Dahisarkar said with extravagant understatement.

His office fined about 30,000 people $12 each last year for breaking rules. Tickets average about 10 rupees (25 cents).

Last year, 186 people died when bombs went off on seven trains during the evening rush hour.

The British built the first railways in Mumbai in the 1850s, but more recently a lack of investment and a city growing rapidly as migrants flood in has stretched the system to breaking point.

Officials are splashing out on a $2 billion upgrade -- the first major overhaul since independence in 1947 -- that will add 180 km (110 miles) of new track and 147 trains over five years.

But even when all these new coaches are rolling, commuter trains will have to carry 1.5 times their capacity at peak hours.

HERDED LIKE GOATS

For Mumbai's 17 million residents, the railways are indispensable but a form of slow torture: travelling from the southern business districts to the ever-lengthening northern suburbs where most live takes about two hours.

Road travel is also painfully slow and often impossible during the annual monsoon, when torrential rains flood streets waist deep.

Mumbai's stations are busy round the clock, as waiting passengers, new arrivals with one way tickets and looking for work, the homeless, beggars, drug addicts, dogs and cows battle for space.

Trains -- plastered with adverts for items like toothpaste, underwear and condoms -- run every five to seven minutes, but delays are common, leading edgy commuters to hit out at the slightest provocation.

Angry passengers riot a few times each year, rampaging through stations and, occasionally, torching trains and clashing with police.

"We feel like goats and sheep herded into a compartment with no space to move and no proper ventilation," said 52-year-old shopowner Rajesh Vasavda.

"I bet no high-level government official ever takes a ride back home in these crowded trains. The day they are forced into one, I am sure immediate action will be taken."

Authorities say the ongoing network upgrade will improve matters, but commuters are sceptical.

"Plans are always made, but nothing really happens," said Bidisha Mukherjee, a young commuter. "If they want to do something, it had better be quick."

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