Half of India’s 1.2 billion people were without power on Tuesday as the grids covering a dozen states broke down, the second major blackout in as many days and an embarrassment for the government as it struggles to revive economic growth. Stretching from Assam, near China, to the Himalayas and the deserts of Rajasthan, the power cut was the worst to hit India in more than a decade. Trains were stranded in Kolkata and Delhi and thousands of people poured out of the sweltering capital’s modern metro system when it ground to a halt at lunchtime.