India's financial capital, Mumbai, has taken on its biggest-ever infrastructure project to build a subway in a city where more than seven million commuters cram onto its creaky suburban railways. Video: Karan Deep Singh/WSJ

Play Video ‘You Have to Actually Cut Open Mumbai’s Belly’—Inside One of the World’s Most Audacious Transit Projects More than 8,000 workers keep construction going 24 hours a day to finish the line that will speed 1.6 million riders beneath one of the world’s most crowded cities