After 75 years of service, Mail Rail, a miniature underground railway stretching across seven miles of the city, is being put up for sale by Royal Mail. The service, the only remaining underground mail railway in the world, shifts 3.4 million letters a week, half the number it carried 50 years ago. The railway, thought to be worth more than £15 million, could be redeveloped as a way to transport goods to London’s busiest streets and shops.