Every schoolday for six adolescent years I used to travel by the Metropolitan, Bakerloo and finally District (or Circle) Line from Northwood in Middlesex to Blackfriars. My school was in the heart of London, its main steps across from the Thames; there was just a short walk to St Paul’s for the annual commemoration of our benefactors. This transition from calm, green suburbia to vibrant metropolis felt in the main a simple psychological process: from family dullsville to the centre of the world. But there was one thing—one building—one part of one building—which usefully complicated such daily world-turning.