The Camden Rat Hole is a service tunnel, forming part of the London Underground and was used as an empty carriage line.
Brief History of the Camden Service Tunnel
The Camden Service Tunnel, often called the Camden Rat Hole served as an empty carriage line between Willesden and Euston. It runs from south of Primrose Hill to where it connected to the Broad Street line, hidden beneath mainline tracks that run directly above. The distinctive sound of trains running overhead can be heard while inside the tunnel – it always amazes me how much can be heard while deep under the streets of London.