I would like to highly recommend a visit to the Postal Museum and Mail Train in London.
We did it today. Have a look at the videos and photos below.
The website with all the details is here.
In the earlier 20th century the managers of the Post Office were concerned with the congested traffic on the roads of London. This was impacting on their ability to deliver the post. Their solution was to build a network of underground tunnels connecting Paddington station, to the west, Mount Pleasant Mail Centre, in the Centre, and Liverpool Station to the east. They then added miniature electric trains to carry the mail bags. The Post Office now lets you experience the route the mail used to travel.