Where does the wind on the tube come from?

I am physicist-turned-science-writer, whose book, Science and the City explores the engineering and technology that keep the world's great metropolises moving. As part of my research, I was lucky enough to spend time with the team behind Crossrail, and make some videos. In this one, I talk about the piston effect, which is used to ventilate deep-level train tunnels. Below is an excerpt from Ch6 of my book, which explains where it comes from - enjoy! "I learned about the piston in my first week of living in London. As I emerged onto a busy platform at Clapham South, a huge gust of wind ripped off my scarf and lifted my dress clean above my head. I've never quite got over the embarrassment. So, what causes it? Out in the open air, as trains move they easily push air molecules aside in all directions. But in tunnels, that air is confined, so it can't get out of the way quite so easily. This is especially true for train tunnels that are only slightly larger than the trains travelling through them. Just like a piston in a car engine, as a train moves through an underground tunnel, a wall of air molecules builds up in front of it, producing an area of warm, high-pressure air. Behind the train, you get a corresponding area of low pressure – so low that it can even act as a mini-vacuum, sucking the train back. Add to that the fact that many metro stations have areas above ground, or open out into the street, and you end up with a turbulent exchange of air. A wind that is unique to the underground, and one remarkably skilled at embarrassing people. Although it sounds destructive, the piston effect can help ventilate train stations – in fact, it' s often the only form of ventilation." .... to read more, you can find Science and the City in all good bookshops or wherever you download your books from.