Dan Snow, Anita Rani, Robert Llewellyn and John Sergeant travel to India to explore the science behind the systems running the world's busiest railway from the Chhatrapati Shivaji Terminus in Mumbai. The team meets the people who keep the large transport structure organised and on time. However, even on a good day, the commuter trains have up to 14 people crushed into a square metre; Snow, Rani and Llewellyn experience this first hand as they travel in carriages twice as packed as most busy UK trains. Elsewhere, Sergeant discovers more about India's historical connection with teas and trains at Darjeeling's steam-powered hill railway.