The Myers-Briggs Type Indicator assessment is a set of questions, to which someone’s answers will supposedly tell you exactly what kind of personality they have. For example, you can find out whether they think about things, or perhaps have feelings.

The test was created by mother and daughter Katharine Cook Briggs and Isabel Briggs Myers, fans of psychologist Carl Jung, after Katharine noticed that her son-in-law had a different personality to the rest of her family, something that definitely required formal and rigorous investigation.

Someone’s Myers-Briggs Type is represented by a sequence of 4 letters. You sometimes see these on online dating or social media profiles: “I’m an INFJ”, someone might say, which means that they’re "Introverted, Intuitive, Feeling and Judging". The opposite would be an ESTP: "Extroverted, Sensing, Thinking and Perceiving". Some famous ESTPs, according to my extensive research, include David Cameron, Hermann Goering, Madonna and the Circle Line.

You might think it would be hard to judge the personality type of the Circle Line, because it’s a tunnel in London with some trains in it, but you’d be wrong. All the tube lines have Twitter accounts these days – and whilst the normal way to establish a Myers-Briggs Type is get someone to fill in a questionnaire, it’s apparently possible to use a sample of text to analyse the personality of the author.

So, in 2013, I ran an analysis on all a collection of the tweets that have been posted by each London Underground line – and here’s what I found:

Almost every tube line is either ESTJ (a personality type sometimes described as "The Doer") or ESTP ("The Guardian"). There's just one exception: the Waterloo & City Line is ESFP – "The Performer" – a personality type shared by Miley Cyrus.

That the Waterloo & City lines has a fundamental difference in personality from its peers seems to be borne out by looking at how the different tubelines interact. Here's a slightly confusing diagram showing how often the different tube lines mention each other on Twitter. The redder the boxes, the less "social" a line is:

Lines tweeting on the horizontal, lines being tweeted at on the vertical. Samples of around ~3200 tweets per line.

The Waterloo & City Line is by far the least social. The other lines just aren’t interested in it, and it isn’t particularly interested in them. But why is it so different?

The line opened in 1898, built by the London & South Western Railway, because even after they’d gone to all the effort of moving their terminus from Vauxhall to Waterloo, that still didn’t actually get most commuters to where they worked in the City. It’s the only London Underground line that’s completely underground (taking trains on and off the line used to be done using a hydraulic lift; nowadays they use a crane). But for a long time, it wasn’t really a London Underground line at all. It doesn’t even appear on Harry Beck’s original 1933 tube map, despite pre-dating it by over 30 years.

That it opened as an independent railway line isn’t unusual among the older London Underground lines. What is unusual is that it was still operated by Network South East a national rail line until 1994, when it was decided to correct the anomaly before rail privatisation took place. For arcane contractual reasons, London Underground bought the line for a pound.

The 1987 tube map does include the W&C, but it’s marked as a National Rail service.

Once integrated into the tube network proper the line was given the exciting colour of turquoise, all the good colours having already been used up. At least it’s better than what the Jubilee Line has been known to claim is “silver”. Chinny reckon.

Maybe that long isolation from the “proper” underground lines has taken its toll, marked the Waterloo & City out as different. The sad thing is, it seems like it’s trying to be chatty and approachable: it may be the line that tweets the least but, if we look at how much each line tweets given the number of stations on it, or the total line length, the Waterloo & City has them all beat hands down. It’s also the least egotistical line, mentioning itself less than any of the others.

Whoa, District line, get over yourself.

If we look at the words it uses the most, we can see while it’s pretty work-focussed, it is a fan of Easter and the Paralympics.

Perhaps it’s the case that we can’t learn absolutely everything about a thing by analysing its Twitter account. Perhaps. So I took my second ever trip on the Waterloo & City Line, making the journey in the order the name suggests. That I’ve only used it twice in 10 years might not be not that uncommon, as trips on it account for less than 1 per cent of all Underground journeys made in London.

But for what it is, it’s hard to fault: at around 9 minutes (including getting to the platforms) it’s still the fastest way of making the journey between Waterloo and Bank, if you don’t fancy cycling really fast or taking a helicopter. And it’s definitely the most efficient bit of tube line naming on the whole system:

The line’s name was actually even more functional when it was opened, as until 1940 the terminus in the City was called City.

For some reason, on the day I took my trip, every single advert displayed in the carriage was for a special brand of Beats by Dre headphones aimed at executives. Not even a single one for that hair clinic. All Beats, as far as the eye could see.

As my trip was on a weekday afternoon no-one much was about which is good because people might have thought I was a bit of a weirdo for taking photos of Beats by Dr Dre adverts.

At the other end I walked up what as far as I can tell is the tunnel that gave the line the nickname “The Drain”. I have never heard anyone actually call the Waterloo & City Line “The Drain”, but it says they do in about four different books, so it must be true. Anyway, I guess this tunnel is a bit like a drain, if drains had lights and steps? I’m not going to start calling the Waterloo & City Line “The Drain”.

Later that day I found a book in Guildhall Library which is mainly about 200 pages of Victorians arguing about corners but does have some exciting fold-out diagrams of the Waterloo & City Line – handy for if you want to want to build a Waterloo & City Line. Here’s a very bad photograph:

To be honest, I didn’t really learn a lot about the Waterloo & City Line from this experience. I think it’s just some trains? The tweets are almost certainly just written by someone in an office, and this has probably been a largely pointless exercise.

But then the Myers-Briggs test is itself a largely pointless exercise, taken seriously only by people who work in HR and designers of bad online quizzes, where you can find out if you’re more or less perceptive than Clara from Doctor Who. So let’s call that the point and move on with our lives.

Ed Jefferson writes some things on the internet. He tweets as @edjeff.

An earlier version of this article appeared on his blog, Some Fool Notion, in September 2013. It's reposted here with his permission.