Bet you've never noticed this about the Tube map

Why do some lines go over and some go under?

Much celebration today at the unveiling of a blue plaque to Harry Beck, inventor of the cultural icon and design classic that is the London Tube map. You don't need me to tell you about his genius insight that the map didn't have to be geographically accurate: that's in all today's news reports. (If you want to see a pre-Beck map, there's a fantastic original in a glass case outside Temple station.) Instead let me share my favourite little quirk about the map as it appears today.

Bet you've never noticed this about the Tube map

There are two sorts of Tube line: ‘cut and cover' (the early ones, like the Metropolitan and District, built by digging up the road, putting down the tracks then relaying the road), and ‘deep level' (made by tunnelling much further underground, for instance the Piccadilly, Northern and Central lines).

Take a look at the Tube map: whenever a cut and cover line crosses a deep level one, it goes over rather than under it, to mimic what happens in reality. In the 1980s London Underground accidentally printed a version of the map where a deep level went over a cut and cover. Beautiful pedants that Londoners are, several wrote to point out the mistake. A new version of the map was duly issued.

Now that's what you call attention to detail. I bet old Harry Beck is smiling down in approval.


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Big Ben

On a visit to Big Ben, I was told that if you stand at the bottom of the tower with a portable radio and listen to the chimes on Radio 4 (they still transmit them live), you hear them on the radio before you hear them ‘for real’. I couldn’t believe it – but was intrigued enough to try it for myself. And you know what? It’s absolutely true. The bongs come out of the radio a fraction of a second before they reach your ears from the top of the tower. It’s something so silly, so counter-intuitive, that you have to tell people. (Well, I did.) Researching the explanation, I found that it’s because radio waves travel at the speed of light (186,000 miles per second) rather than the 700 or so miles per hour at which sound waves travel. The signal travelling down the wire from the microphone to the BBC goes at the speed of light too. Hence the radio version overtaking the real one.

I realised that this would be the perfect way to teach the principle in school physics lessons. Instead of a boring teacher droning on that ‘radio waves travel at the speed of light’, illustrate it with this beautiful and quirky little fact. The kids will remember it then. I certainly would have done if my physics teacher had taken this approach. As it was I had to wait until I heard a piece of so-called ‘trivia’ in my thirties.