If you travel on a Thameslink train between Tooting and Streatham, you can have, from the left-hand windows as you face forward, a rare view of old London. It can be glimpsed only very briefly, and I would probably never have noticed it if a signal had not one day brought my train to a dead stand with my seat in just the right position, so that I was presented with the scene for as much as two minutes rather than perhaps two seconds.
Between the two stations the train travels at rooftop level for some distance, and there is a stretch where the only buildings to stick up above the houses are churches, some old London board schools – and one modern block. The houses are initially a mixture of late nineteenth century terraces, inter-war and modern, but then we leave the inter-war and modern houses behind. Some more modern houses will appear less than a minute later, but for that brief time we see only the late Victorian terraced houses. A few seconds into that short time parallax causes one of the board schools momentarily to blot out the modern block (on the horizon, left) so that, TV aerials and suchlike apart, we glimpse a roofscape of gables, tiles and chimneys which has barely changed for at least a hundred years – or we would if someone in the foreground had not had a loft conversion done since I first noticed the phenomenon.