The Hammersmith and City line appeared on the Tube map as part of the Metropolitan line until 1990, from then on it became a separate line.
The twins' line starts from the sheds where they go directly to the Industrial Estate Station. After this, it arrives at International Underground Station, before continuing onto the funfair. After a quick loop-de-loop, they arrive at the school and after passing through International Underground again, the line terminates at the sports stadium.
In real life
The Hammersmith and City line runs between Hammersmith and Barking. Between Farringdon and Aldgate East it crosses the financial heart of London. Over 114 million passenger journeys are made on the Circle line and Hammersmith and City line combined every year.
http://en.wikipedia.org/wiki/Hammersmith_%26_City_line - for information on the real Hammersmith and City line