You may never have used it. You may not even know that it’s there. But in zones one and two of the London Underground network, you’ll find an oft-forgotten piece of London’s transport history.

The Northern City line is a six-stop underground route from Moorgate to Finsbury Park. (It’s officially, if confusingly, known as the Moorgate line.) But, unlike other underground lines, it not part of Transport for London’s empire, and is not displayed on a normal tube map. Two of the stations, Essex Road and Drayton Park, aren’t even on the underground network at all.

The line has changed hands countless times since its creation a century ago. It now finds itself hiding in plain sight – an underground line, not part of the Underground. So why exactly is the Northern City line not part of the tube?

The Northern City line, pictured in dotted beige. Source: TfL.

As with many so many such idiosyncrasies, the explanation lies in over a century’s worth of cancellations and schemes gone awry. The story starts in 1904, when the private Great Northern Railways, which built much of what is now the East Coast Main Line, built the line to provide trains coming from the north of London with a terminus in the City. This is why the Northern City line, unlike a normal tube line, has tunnels wide enough to be used by allow mainline trains.

Eventually, though, Great Northern decided that this wasn’t such a bright idea after all. It mothballed plans to connect the Northern City up to the mainline, leaving it to terminate below Finsbury Park, scrapped electrification and sold the line off to Metropolitan Railways – owners of, you guessed it, the Metropolitan line.

Metropolitan Railways had big plans for the Northern City line too: the company wanted to connect it to both Waterloo & City and Circle lines. None of the variants on this plan ever happened. See a theme?

The next proposed extensions, planned in the 1930s once London Underground had become the domain of the (public sector) London Passenger Transport Board, was the Northern Heights programme. This would have seen the line would connected up with branch lines across north London, with service extended to High Barnet, Edgware and Alexandra Palace: essentially, as part of the Northern line. The plans, for the main part, were cancelled in the advent of the Second World War.

The Northern Heights plan. The solid green lines happened, the dotted ones did not. Image: Rob Brewer/Wikimedia Commons.

What the war started, the Victoria line soon finished. The London Plan Working Party Report of 1949 proposed a number of new lines and extensions: these included extension of the Northern City Line to Woolwich (Route J) and Crystal Palace (Route K). The only one of the various schemes to happen was Route C, better known today as the Victoria line, which was agreed in the 1950s and opened in the 1960s. The new construction project cannibalised the Northern City Line’s platforms at Finsbury Park, and from 1964 services from Moorgate terminated one stop south at Drayton Park.

In 1970, the line was briefly renamed the Northern Line (Highbury Branch), but barely a year later plans were made to transfer it to British Rail, allowing it to finally fulfil its original purpose.

Before that could happen, though, the line became the site of a rather more harrowing event. In 1975, the deadliest accident in London Underground history took place at Moorgate: a southbound train failed to stop, instead ploughing into the end of the tunnel. The crash killed 43 people. The authorities responded with a major rehaul of safety procedure; Moorgate station itself now has unique timed stopping mechanisms.

The last tube services served the Northern City Line in October 1975. The following year, it reopened as part of British Rail, receiving trains from a variety of points north of London. Following privatisation, it’s today run by Govia Thameslink as part of the Great Northern route, served mainly by suburban trains from Hertford and Welwyn Garden City.

Nowadays, despite a central location and a tube-like stopping pattern, the line is only really used for longer-scale commutes: very few people use it like a tube.

Only 811,000 and 792,000 people each year enter and exit Essex Road and Drayton Park stations respectively. These stations would be considered the fifth and sixth least used in the tube network – only just beating Chorleywood in Hertfordshire. In other words, these usage stats look like those for a station in zone seven, not one in Islington.

One reason for this might be a lack of awareness that the line exists at all. The absence from the tube map means very few people in London will have heard of it, let alone ever used it.

Another explanation is rather simpler: the quality of service. Despite being part and parcel of the Oyster system, it couldn’t be more different from a regular tube. The last (and only) time I used the line, it ran incredibly slowly, whilst the interior looked much more like a far-flung cross-country train than it does a modern underground carriage.

Waiting for Govia. Image: Haydon Etherington.

But by far the biggest difference from TfL is frequency. The operators agreed that trains would run between four and six times an hour, which in itself is fine. However, this is Govia Thameslink, and in my experience, the line was plagued by cancellations and delays, running only once in the hour I was there.

To resolve this, TfL has mooted taking the line over itself. In 2016, draft proposals were put forward by Patrick McLoughlin, then the transport secretary, and then mayor Boris Johnson, to bring "northern services... currently operating as part of the Thameslink, Southern and Great Northern franchise" into TfL's control by 2021.

But, in a story that should by now be familiar, Chris Grayling scrapped them. At least it’s in keeping with history.