Leinster Gardens is a street like any other in affluent west London. Moments away from the expanses of Hyde Park, and just a stroll away from Bayswater and Queensway Tube stations, it’s lined with tall, elegant white houses, formally arranged cheek by jowl, their stucco-fronted façades endlessly seeming to preen themselves for the admiration of passers-by.

Some of the buildings – many now taken up by hotel rooms – have Range Rovers parked outside; others sport a Porsche. The lowliest boast merely an Audi with a glimmering chassis fighting the freshly-painted black wrought iron railings in a battle to be the shiniest.

This affluence – which has been a feature of the neighbourhood ever since these streets were first laid out in the 1840s – is also the reason for its well-hidden blemish.

Stroll along past numbers 23 and 24, and you’ll notice something odd.

23 and 24 Leinster Gardens. Image: Google Maps

Where windows in other houses give glimpses of vast subterranean kitchens with exorbitant puddles of orchids on every surface, opaque grey-tinged panes block any view into 23 and 24. The doors, in a slightly less well-kept black, have no handles.

These houses don’t exist.

Behind the crisp, white façades, the space is occupied by a gaping void, occasionally filled by a passing Circle or District line Underground train.

This stretch of railway – an extension of the Metropolitan Railway’s original stretch between Farringdon on to South Kensington – was approved by Parliament in 1864, and obviously, at that point, harboured steam trains, puffing their way through the cuttings and tunnels between South Kensington and line’s newly-extended terminus at today’s Tower Hill.

The back view of the false facade, from Portchester Terrace. Image: Google Maps.

And even though the trains were fitted with condensers, large vents were still needed to give what remaining steam there was a way out.

These openings can be found along the earliest tube lines, if you know where to look – and if you look at it from above.

Just east of South Kensington, between Walton Street and Donne Place, the polite houses on both back onto an exposed stretch of the old District Railway.

The opening between Walton Street and Donne Place. Image: Google Maps.

A little further east still, between Halsey Street and Rawlings Street, the same happens again.

An aerial view of Halsey Street and Rawlings Street. Image: Google Maps.

South-east of Sloane Square station, the line comes up for air again between Graham Terrace, Eaton Terrace and Ebury Street.

A jaunty view of the cutting by Eaton Terrace. Image: Google Maps.

On a different stretch, between King’s Cross St Pancras and Farringdon, the line runs in the open between St Chad’s Place and Wicklow Street, ducking under cross streets and below the backs of terraced houses.

The cutting near King's Cross is likely longer as the neighbourhood was less affluent. Image: Google Maps.

Because of the way these early lines were run, these spaces, open wounds in the middle of crowded terraces, were vital to make the thing work.

But of all of these, Leinster Gardens is unique. Because of the affluence or the existing neighbourhood, and presumably the NIMBY expertise of the then residents, false matching facades were constructed to replace the two houses that were demolished.

And now we’re left with the legacy of the strange empty houses at 23 and 24 Leinster Gardens, the addresses that don’t exist.

London Underground manages the facade and is responsible for painting the doors and generally keeping it in shape.

It’s a textbook London curiosity, and it’s been used to great effect.

The series three finale of the BBC’s Sherlock used it as a location, and saw Mary Watson’s face projected onto the false facade of the two houses. Bit weird, if you ask me.

According to Andrew Martin, whose book Underground, Overground is something of a bible on this kind of anorakery, practical jokes abounded. Coal merchants sent apprentices to deliver heavy loads to the houses, and letters were addressed to Mr N.O. Body, 24 Leinster Gardens.

He includes a particularly amusing anecdote from when he once visited the two hotels that still sit either side of the void, the Henry VIII and the Blakemore, and asking about the strange fake houses between the two. “Within ten minutes, staff members from each hotel were standing in front of numbers 23 and 24 and saying to each other, ‘But we thought they were part of your hotel’.”

London, eh? Weird place.

Jack May is a regular contributor to CityMetric and tweets as @JackO_May.

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