London's Underground rail system was originally powered by steam engine, but now runs on electricity. Yet there was a third alternative — pneumatic power.
Various systems were built to either push or pull a carriage through a tunnel using a differential in air pressure. Such a set-up carried mail beneath London in the second half of the 19th Century, and a short-lived passenger line operated alongside Crystal Palace Park (pictured above). Most ambitiously of all, a line beneath the Thames between Whitehall and Waterloo was partly constructed in the 1860s, but abandoned for financial reasons.
This little-told story has now been brought together in a wonderful little ebook by Ian Mansfield, of Ian Visits fame. You can read it in installments over on his site, but we'd really recommend buying the book itself to have everything in one place (and to give Ian a few pennies by way of appreciation for his research efforts). It's only £1.53 for Amazon Kindle and compatible devices.