Question: Where two lines cross on the tube map, which line goes on top and which goes underneath?
Hypothesis: The map reflects real life, so the higher of the two lines appears on top.
Let's test that hypothesis. It would be a cracking fact if it were true.
Let's start here.
And let's start by looking at the Hammersmith & City and Circle lines. From left to right these are shown passing above the Bakerloo, above the Jubilee, above the Northern, above the Piccadilly and above Thameslink. All of these are correct because the sub-surface lines run closer to street level than the deeper tube lines. It's a promising start.
Elsewhere the Central line is shown passing above the Jubilee line and above the Northern line. This is also correct, as you'll know if you've tried interchanging via the escalators at Bond Street or Tottenham Court Road. These two stations currently have one blue blob and one white blob, otherwise we'd never know which line went over another, indeed at several other central London interchanges it's impossible to tell. But still, all good so far.
At King's Cross St Pancras it is indeed true that the Piccadilly line platforms (21m below street level) are higher than the Northern line platforms (27m). But Thameslink doesn't pass that low down, so that's spatially incorrect, and the crossing of the Central line at City Thameslink is incorrect too.
Meanwhile near Swiss Cottage it is true that the Metropolitan line passes over the Jubilee line and the Overground, but it's not true that the Jubilee line passes over the Overground. So far every crossing of two Underground lines has been 'correct', whereas Thameslink and the Overground aren't following the rules.
Alas the Underground stops following the pattern here.
Again the Circle line is correct, passing over the Victoria, Jubilee, Northern and Waterloo & City lines. Thameslink is very wrong at Blackfriars, but we'll overlook that for now. The incorrect Underground layering is at Waterloo where the map has Northern over Jubilee over Bakerloo, whereas the real life ordering would be Bakerloo (16m) over Northern (21m) over Jubilee (29m). The Jubilee line has by far the lowest platforms but that's not how it's been depicted. This unusual layering first appeared on the tube map in 2006 - the year step-free blobs first arrived - and has been spatially incorrect ever since.
The other incongruities are at the western end of the Central line.
The tube map shows the Central line passing over the Piccadilly line twice, once between Hanger Lane and North Acton and once between Ealing Broadway and West Acton, But in reality the Piccadilly line passes over the Central line both times, so that's two more separations from reality. The first appearance of this anomaly was in 2007 - the year the Overground first arrived - and it's been spatially incorrect ever since.
As far as I can tell the other two dozen Underground crossings are all correctly layered, i.e. with the more elevated of the two lines appearing on top. But Waterloo and Ealing break the pattern and have done for the last 15 years. I've checked some older maps and everything seemed fine before that.
Conclusion 1: Tube map layering no longer reflects real life, not quite, although it did before 2006.
Conclusion 2: The misbehaving Underground crossings are the Jubilee over the Bakerloo at Waterloo and the Central over the Piccadilly in Ealing. All the other crossings are 'correct'.
But once you start looking beyond the Underground, spatial correctness is mostly abandoned. Instead there seems to be a hierarchy with the Underground at the top and Thameslink at the bottom. The Underground always appears as the top layer when it crosses another mode, whereas Thameslink always passes underneath.
Here in north London, for example, the Northern line runs in deep tunnels but is instead shown passing above Thameslink and the Overground. This is because the hierarchy says the Underground must always appear on top. This is fine, indeed for the sake of legibility and clarity it's probably ideal.
After the Underground in the hierarchy comes the DLR because that's always on top of everything else, as you can confirm at Shadwell and Stratford. Next comes the Overground, which beats everything except the Underground and the DLR, and then comes TfL Rail. The trams are probably next, although they don't cross enough other lines to be sure, and bottom of the heap is the new (and maybe temporary) Thameslink.
Best ignore the Dangleway, although technically it flies above the DLR.
Conclusion 3: The tube map's layering hierarchy is as follows.
Underground
DLR
Overground
TfL Rail
Trams
Thameslink
Conclusion 4: On the tube map the Overground is always underneath the Underground.
It's worth mentioning that where the Overground crosses the Overground - as it does in Tottenham, Walthamstow, Hackney and Bethnal Green - the map always shows the higher of the two lines on top, so some spatial element remains. But once you go beyond the Underground it's more about how important a line is, not its relative elevation.
Hypothesis disproved: The layering of Underground lines mostly matches vertical reality but with a handful of exceptions. For all other modes a fairly strict layering hierarchy exists instead.
Let's finish off with the most surprising discovery from my research, which can be seen on the extracts below.
On the left is the latest digital tube map.
On the right is a scan of the latest paper tube map.
Look at the Central line between Ealing Broadway and West Acton. On the digital map it passes over the Piccadilly line, but on the paper map it goes underneath!
Also look at TfL Rail to the east of Acton Main Line. On the digital map it passes under the Overground twice, but on the paper map it goes over the top!
At Ealing Broadway the paper map matches reality. But at Acton Main Line the digital map is spatially and hierarchically correct and the paper map is doubly inaccurate.
And because it's important to do your research properly, today I've been out to check the poster map at stations. Inexplicably it's different to both of the other two.
On the Central line the digital and poster maps match but the paper map doesn't. On TfL Rail the paper and poster maps match but the digital map doesn't. That's most peculiar.
Somehow the layering of the three maps has become divergent, so that the digital tube map, the paper tube map and the poster tube map are no longer consistent. I haven't spotted any similar oddities elsewhere, but these variations are astonishing enough. It's not necessarily a mistake, but at the very least it's a highly unexpected disparity.
Conclusion 5: The digital tube map, paper tube map and poster tube map are layered subtly differently.
Conclusion 6: If there are any rules to the layering of lines on the tube map, the designers haven't applied them consistently.
