The Arnos Park Viaduct Mystery

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Arnos Park Viaduct on the Piccadilly Line is the biggest brick viaduct ever built for London’s underground. 3 million bricks were used in its construction.
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Arnos Park viaduct and the Pymmes Brook.
Its possible some will point out other viaducts of note on the system however these are simply existing railway infrastructure that was taken over by the tube. The comparable Golders Green viaduct on the Northern Line (built 1922, approx 24 arches/spans) is just half the length of Arnos Park’s as well as being much lower in height.

Every website that features this splendid viaduct claims it has 34 arches (or possibly spans) but is that right?
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The viaduct curving through Arnos Park, these are arches 11, 12, 13, 14 etc. The relieving arches are clearly visible.
The London Underground Railway Society (LURS) quite sensibly says the viaduct has forty of these. Its clear the LURS were thorough in counting the arches as I was. They carefully counted the numbers. But even that missed one arch. Perhaps it was an oversight?
It is clear any claims the viaduct totals thirty-four arches are quite conservative, but are forty arches the correct number? It depends on how one goes about counting the whole viaduct.
A number of the arches have plates with numbers on them. Many are not obvious because of foliage or being rusty and quite a few missing. I did a thorough task logging these numbers and each arch/span.
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The ubiquitous number one!
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Number 19 is by the main pathway through the park.
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22 is where the viaduct begins to run alongside the Pymmes Brook.
The stretch of viaduct right through Arnos park as far as Waterfall Road, N11, clearly has 33 numbered arches. There is no doubt about that. The blind arch right next to 33 to makes it a total of 34. Hence the often touted claim of 34 arches.
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The last full numbered arch before the blind arch at 34.
Number 34 is a blind arch, yet its only about a third the size of the other two blind arches further north. Its not even numbered. This confuses things because the number 36 further north is ambiguously placed on the arch and therefore one is not sure how the arches should be counted between 33 and 36. The position of 36 should be on the other side of the arch it is on, yet its position means one blind arch is left out.
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End of the 34 arches. This section most see as being the Arnos Park viaduct but there’s the other section to consider. The dark arch in the middle of the picture is the blind arch denoted as 34.
One problem as I see it is the name of the viaduct itself. As ‘Arnos Park viaduct’ it seems most ignore the bits outside the park. There’s another bit of park beyond the 34 arches across Waterfall Way with more arches. This is a section of parkland known as Waterfall Walk alongside the Pymmes Brook.
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The Waterfall Road overbridge (number 35) as seen from further up the hill towards Southgate Green.
Can a bridge be part of a viaduct? Essentially an arch within a viaduct is a bridge, and a viaduct is essentially a structure that consists of a string of bridges. Ultimately it does not matter whether a viaduct has brick, concrete, iron, timber, steel, arches or a mix. Some viaducts don’t even have arches in the traditional sense, like the modern Eurostar crossing over the Medway, or Meldon on the former Southern route to Plymouth/Padstow.
Since the original numbering scheme used by the tube clearly includes overbridges, its somewhat a surprise as they include two blind arches but not three.

  1. Many think there’s only 34 arches as the bit beyond the park gates gets discounted.
  2. LURS say its forty (two overbridges included) yet they exclude one blind arch.
  3. The old and newer numbering systems make it more confusing.

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A very rusty 36 by the Waterfall Road roundabout. But which arch does it refer to?
If one were to take it pedantically, the arches (counting the one blind arch, 34) makes the entire lot 38 arches. That is the final number as shown below. The problem with that is what one should make of the other two blind arches and the Hampden Way overbridge? Especially when it is clear the Waterfall Road overbridge is numbered.
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Number 38 is the very last number on the entire viaduct. The blind arch behind this is not numbered.
The ambiguous positioning of 36 and 38 means one way you can make it a total of 38 arches, or if you add Hampden Way its then 39. But this excludes the two blind arches on the other side. Does that mean one counts the one small blind arch and excludes 2 far bigger blind arches? Rather an odd way of doing things.
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The northern end of the viaduct. Arches 33 and 34 (the blind arch) are within the park itself at far left. Following these is the Waterfall Road span. Hampden Way’s is just out of sight to the right. There are actually five arches in this section, two are blind.  The last visible arch is 38 and Hampden Way overbridge, immediately adjacent, must be 39.
Because not all the blind arches are counted the numbering becomes a bit murky. On the main section of the viaduct the numbers clearly denote which arch its referred to yet on the bit beyond Waterfall Road the numbers don’t correspond with the arches. Two blind arches are excluded. Yet a further confusion arises if one follows the numbers pedantically it becomes somewhat apparent one of these blind arches are not included. I think the fault lies in sloppy work with the original numbering.
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This is number 39 (the northernmost blind arch.)
What is a blind arch? Well its one that’s normally built to save weight however these are extra deep and built for a different purpose, that is, to give extra strength in anchoring the overbridges. Blind arches are also a cost saving measure yet these are gigantic – clearly not cost saving arches!
Its obvious the old numbers confuse things and it seems those who put the numbers on the viaduct in the 1930s didn’t attempt to correspond them well to each arch after 34. If one counts every single span that exists no doubt its 41.
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The uncounted blind arch just off Waterfall Road. Numbers 35 and 36 are either side of this.
So there you have it! Completely disregarding the logic given by the original numbering, Arnos Park viaduct has 41 arches – or spans if you prefer.
A final note. How does TfL number its bridges these days? There’s a clear system of classification as observed on most of the tube system, and data from TfL confirms this. A viaduct is classified as one component whilst an overbridge is a separate component. For example on the Bow Road to Bromley by Road or Parsons Green to Putney Bridge sections of the District Line, the separate viaduct sections are given one number for the string of arches within that section whilst the overbridges in between are given their own separate number.
PT stands for Piccadilly Tube. The bridge plates usually denote the line itself, D = District, MR = Metropolitan Railway etc.
On the Piccadilly the older scheme of bridge numbering is used. (This is not the original numbering used in the 1930s construction of the viaduct.) The crossing across the North Circular Road is PT71 and PT71a (the latter can only be seen from the train.) I assume the bit of viaduct to Bounds Green Brook is PT72 whilst the bit beyond is PT73. Bowes Road bridge (by the station) is PT74 whilst Arnos Park Viaduct is PT75 as denoted by the tracks (confusingly the start of the viaduct is also classed as PT75A at its base by Arch number one within the park.)
We can see from the current classification used on the Piccadilly Line that the overbridges are indeed numbered separately as a general rule. But I have yet to work out how the logic is implemented.
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The start of PT75A which is right by Arch number one at Arnos Grove station.
Either side of the Pymmes Brook its PT75A. /PT76A. The Pymmes Brook span is PT76 (numbers 26 and 27) whilst PT76A leads to Waterfall Road (also PT76.) It seems to me PT75 counts arches 1 to 26 whilst PT76 only counts arches 27 to 34! Its quite an uneven balance of numbering. Maybe someone can explain?
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PT76 covers the Pymmes Brook to Waterfall Road section.
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Waterfall Road overbridge.
That over Waterfall Road is PT77 and Hampden Way gets PT78. That’s confusing for it appears to exclude the five odd arches between PT77 and PT 78! Are those five arches part of PT77 or part of PT78 – is there an ‘A’ suffix somewhere thats not obvious? That in itself adds more confusion to how the 41 spans of Arnos Park viaduct should be counted.
This post is part of the following series:
85 years of the tube beyond Finsbury Park
85 years of the tube beyond Finsbury Park #2
The Cockfosters Extension 1: Manor House
The Cockfosters Extension 2: Turnpike Lane
The Cockfosters Extension 3: Wood Green
The Cockfosters Extension 4: Bounds Green
The Cockfosters Extension 5: Arnos Grove
The Cockfosters Extension 6: Southgate
The Cockfosters Extension 6: Southgate (2)
The Cockfosters Extension 7: Oakwood

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