the distant echo of faraway voices boarding faraway trains....

Underground railways look the same everywhere in the world.

How different can they be? They have trains, they have tunnels, they have stations and they have passengers. Even the hot wind that heralds the arrival of the next train is the same. What else is there?

Nonsense.

Every single one is different.

Even if everything else about them was the same; if they had the same trains, the same tunnels and the same platforms, then the passengers would still mark each one out as unique. Our cities are so full of people and yet they remain so impersonal, so faceless. It's much the same underground, of course. It's the same, and yet subtly different rules apply and the bubble of isolation that you so carefully maintain above ground is here rendered smaller and more vulnerable to the pressing mass of humanity squeezed all about you.

All of this struck me as I travelled on the Paris Metro this afternoon. I climbed onto the train in a quiet suburban station and rode it all the way through the centre of town and out again as far as the airport. Initially the train was quiet and flew past other sleepy stations on the approach to the city. Once in the city itself though, the train drew to a halt at every station and the doors opened to allow the ebb and flow of the human tide. I don't travel on the undergound in London very often, but enough to notice that things were not the same here.

The most obvious difference was in the way people looked. The underground in London is packed with people from many different walks of life and from many different countries. The Metro in Paris is no different, of course, and yet the people here looked different - alien almost. Superficially they are the same: they come and go, chatting and laughing, listening to their iPods and reading their trashy novels...but look more closely and things are not the same: their clothes looked different, they wore different types of glasses, ate different snacks and lazily tapped text messages into different kinds of mobile phone. Just as in London, there was an enormous ethnic mixture on the train and a wide palate of different skin colours, yet here they somehow seemed to be in a variety of subtly different shades.

Sitting in my seat watching people come and go over the 65 minute journey, it struck me that although this was a very familiar scene, in other ways it was quite alien. People may well be the same everywhere, but we are also incredibly different in all kinds of tiny ways.

It even smelt different.

As I stepped off the train and walked away towards my flight home, I looked back over my shoulder. As the carriage pulled away and continued on down the tunnel, I wondered if anyone in there had been watching and wondering about me too.