Season 4

Tim Dunn and Siddy Holloway explore the unusual station at Old Street, which serves both the Northern line and mainline trains.

Tim Dunn and Siddy Holloway meet at Gloucester Road in London's museum quarter - a station serving the Circle, District and Piccadilly lines.

Tim Dunn and Siddy Holloway explore Stockwell station on the Victoria line.

Tim Dunn and Siddy Holloway explore the maintenance depot at Northumberland Park.

Tim Dunn and Siddy Holloway explore the maintenance depot at the end of the Central line.

Tim Dunn and Siddy Holloway explore the very first tunnel under the Thames.

Tim Dunn and Siddy Holloway get exclusive access to the disused station at South Kentish Town.

Tim Dunn and Siddy Holloway explore the forgotten tunnels underneath one of Britain's busiest stations - Waterloo.

Tim Dunn and Siddy Holloway explore Paddington station to discover the tunnels hiding clues to the recent past.

Tim Dunn and Siddy Holloway explore Earl's Court station where the old and the modern sit side by side.