Odisha train crash: Why do trains in India go off tracks?

India's expansive railway system - one of the world's largest - carries some 25 million passengers every day across a countrywide network of tracks spanning more than 100,000km (62,000 miles). Some 5,200km of new tracks were laid last year, according to Railway Minister Ashwini Vaishnaw. Also 8,000km of tracks were also being upgraded every year, the minister said.