You might not know that there exists an underground train line in London that has never carried passengers.
This line crosses London from East to West, and not many people know that it exists. It is the Mail Rail, the train used by the Post Office to transport large volumes of letters and packages among its main offices.
It was designed as a mail delivery system in the early 20th century and it stopped working in 2003 due to the lower costs of transporting the mail above ground, and as a Royal Mail spokesman said in that year, the service costs more than four times as much due to the huge drop of postage in the city. Sometimes though, there's a train going through the line to keep the system in working conditions.
During its busiest period, these trains transported up to four millions of letters per day. Each train took 26 minutes to go through the 37 kilometers that separated Paddington from Whitechapel, including 1 minute stop in each one of the stations. However, during the 90's, the e-mail sentenced this system of mail transportation to death little by little. It is nowadays inactive but kept in working conditions, although it has an uncertain future.
You can learn much more about the Mail Rail in The Unofficial MailRail Website and also in the following video which briefly tells its history: