The Mumbai Suburban Railway, popularly referred to as Locals forms the backbone of the city's transport system.[57] It is operated by the Central Railway and Western Railway zones of the Indian Railways.[58] Mumbai's suburban rail systems carried a total of 6.3 million passengers every day in 2007.[59] Trains are overcrowded during peak hours, with nine-car trains of rated capacity 1,700 passengers, actually carrying around 4,500 passengers at peak hours.[60] The Mumbai rail network is spread at an expanse of 319 route kilometres.