For as many London Underground tunnels and stations that have been completed over the years, there are probably at least as many that were planned but never built or opened.
These mothballed plans make an interesting alternative history of what the London Underground might have been – the what-ifs and maybes of the network that we all know and love.
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Sometimes these projects had grand ambitions that reflected the times in which they were built.
One such scheme was the so-called Northern Heights plans.
This was to be a network that would expand the Underground and bring together a web of existing London and North Eastern Railway (LNER) overground lines north of Highgate.
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The line would be extended to include London’s northern hills with lines from Edgware to Bushey Heath, a section from Mill Hill East to Edgware, and a line from Finsbury Park to Alexandra Palace with an interchange linking it to the Northern Line at Highgate.
Some new sections would be built and some existing overland routes electrified.
Work got underway on the scheme in the late 1930s and electrification cables were laid along some of the lines ready for use.
But World War II got in the way of the scheme.
A new depot at Aldenham was built but was converted to build aircraft instead.
Work on the other parts of the plan was suspended in 1939.
After the war, the area north of Edgware was made into green belt land and this stopped houses from being built on it.
Demand for stations at Bushey Heath vanished overnight and suddenly the whole Northern Heights plan looked outdated.
Traffic on the Alexandra Palace branch suffered badly through competition with the new electrified High Barnet branch and from buses and cars.
Mill Hill East became the end of a single-track branch instead of a station on a double-track line between Finchley Central and Edgware.
The Northern Heights plan was officially dropped in 1954 as expenditure was cut after the war.
What would have been the main depot for the service at Aldenham was converted into a facility for fixing buses.
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Then the line from Finsbury Park to Muswell Hill and Alexandra Palace via the surface platforms at Highgate was closed altogether to passenger traffic in 1954.
In the end, we’re left with the existing Northern line services to Edgware, Mill Hill East, and High Barnet but there is no London Undeground connection between them.
Part of the line remained in use until 1971 for goods trains and Underground stock movement, but eventually, the tracks were lifted and much of the track’s pathway has since been re-developed.
Almost all of the original trackway from Finsbury Park to Alexandra Palace has been converted into a public footpath and in 1990 what became known as the Parkland Walk was declared a local nature reserve.
Northern Heights would have linked up London’s northern extremities, but like so much London Underground history, it wasn’t to be.
Remains of the network do still exist though.
There are some big holes in the ground which formed part of a half-completed viaduct sitting by the A41 in Edgware.
The platforms of the original high-level station at Highgate which would have been a key link in the network remains in good condition to this day although much of the land around it is now overgrown.
And the line from Finsbury Park to Alexandra Palace remains as a popular walking route.
Enthusiasts still have a lot of fun tracing the original routes.