We all love a secret, don't we? And the Postal Museum's secret railway is one of the best. Mail Rail runs underneath London's streets. It used to deliver mail, and now the 100 year old network delivers delights to museum visitors in its tiny trains. The museum has been nominated as Museum of the Year 2018, and it's only been open a few months - I guess the railway won't be secret much longer.
Some seem surprised that this museum has been such a success, but I'm just as surprised that they're surprised. People have an innate need to communicate, and the story of our postal service is fascinating. It's battled through attacks by highwaymen, bombs and even a lion (it's worth a visit to the museum for that story alone).
The secret railway sign is on the paving of a pretty garden area which includes a living wall studded with red post boxes. For fans of food, the cafe is worth a mention, as it's a notch better than the usual fare. So it's probably worth a visit now, while it's still (nearly) a secret. More info here.