NEW DELHI—India suffered the world's biggest-ever power outage Tuesday as transmission networks serving areas inhabited by 680 million collapsed, putting the nation's ramshackle infrastructure on stark display.

The grid failure, the second massive blackout in as many days, happened around 1 p.m. local time and affected 18 states and two union territories in north and eastern India, grinding trains across large swaths of the country to a halt, forcing thousands of hospitals and factories to operate on generators, temporarily...