There are around 40 'ghost' stations on the London Underground railway. Abandoned due to disuse or the building of newer stations, many of their platforms and parts of their surface buildings can still be seen, if you know where to look. Some have been closed since the Twenties and Thirties, and were used as air-raid shelters during the war. Down Street, near Green Park, was used as Winston Churchill's War Room and several others have been employed by the Ministry of Defence as offices: Aldwych, King William Street and Brompton Road all still show signs of wartime occupation.

The London Transport Museum used to arrange tours around Down St and Aldwych - one of the first stations built, closed in 1994, now restored for the use of film crews - but these have recently been discontinued, so there's now no official access to any of the disused stations, although if you really want to get in there are ways and means. (not that I would of course recommend any illegal trespass- type activities, but I would recommend good digital cameras that can deal with low light). Access to the below-ground stations is difficult and dangerous without a guide. Many of the stations were built before escalators and had lifts: the lift shafts remain for ventilation, but the stairs have often been partially removed. Someone tried to get into Brompton Road, which had not been entered for over 40 years, and is MoD property. Their body was found at the bottom of a 120-ft lift shaft a month later.

Spotting the ghost stations underground, from the safety of the train, is fairly easy when you know where they are. Stare at the walls and the cable will suddenly vanish. At first you'll just see a gap in the blackness: gradually, you'll begin to pick out the archways of tunnels and steps and the remains of tiling. My regular route on the Northern Line has two, South Kentish Town, between Kentish Town and Camden Town, and City Road, between Angel and Old Street. South Kentish Town closed in 1924. The surface building (designed by Leslie Green, who's responsible for the traditional redbrick arched and tiled stations) is still there, and you can clearly see the space where the platform was underground. Trained eyes can spot the entrance to the stairs, still tiled. City Road, built a couple of years earlier and closed in 1922, has a short platform, built before a greater number of users necessitated lengthening work on the rest of the stations. The only remaining part of the surface station is the lift shaft, left to provide ventilation. Visible a short walk away near King's Cross is York Road Station, another original red-tiled station. The building is fully intact, surrounded by low fencing. It closed in 1932. The platform can be seen between King's Cross and Caledonian Road on the Piccadilly Line.

Stations with above-ground platforms are easier to get a look at. The easiest to see is Wood Lane, in front of Broadcasting House (the BBC HQ) in West London. It was built to serve the 1908 Franco-British exhibition at White City, and closed in 1947. It's imminently scheduled for demolition. Inside, some of the old signs and posters are still visible. In North London, near where I live, is the old Highgate High Level station, hidden in a cutting on the Archway Road side of Highgate, remnant of a line across North London from Finsbury Park to Alexandra Palace. It's mostly still intact, although overgrown. At one end twin tunnels, boarded up, lead towards Crouch End. A spooky passage leads down into darkness, to a locked gate and a blocked exit which goes nowhere (behind the wall is the current station's booking office - they were originally connected). There are no signs or posters. There's a post-apocalyptic feel to it: you can imagine yourself in an abandoned city, overgrown by thickets of trees and trailing ivy.

Some amazing pictures, a more comprehensive list of stations, and some good background information live at Hywel Williams's excellent site: