In an industrial patch of Central London no tourist usually wanders into, you can now find two new, well-done attractions in one, and both are perfect for kids. The first holds exhibits about the social history of mail delivery in the country, from origins (Henry VIII was once the only guy allowed to get mail) to antique vehicles to old “pillar” post boxes to Arnold Machin’s sculptures of the young Queen Elizabeth II photographed to make stamps. There are lots of things for kids to press and bang, including a pneumatic tube that sends messages whizzing in an overhead tube across the hall, and there’s a mail-themed play space, which costs a little extra. But the most memorable aspect of a visit will be a 20-min. loop on the Mail Rail, a little-seen 22-mile underground line that from 1927 to 2003 was used to deliver millions of letters a year. A short portion (with recorded narration and a plastic dome over the carriages, which are as tiny as the train in the kiddie section of a carnival, so please refrain from being tall) now carries curious tourists on a 1-mile looping journey 70 feet below the streets. (Claustrophobes can see what riders do by getting an Exhibitions Only ticket and watching what they missed on screens near the boarding platform.)