Railway Mania
Parliament let anyone propose a railway, newspapers ran pages of prospectuses, and the middle classes — clergymen, widows, Charlotte Brontë among them — poured savings into schemes for lines that were never surveyed and often never built. Investors paid a deposit and owed the rest on call; when the calls came and the money was gone, they were ruined. At the peak, proposed railways would have cost roughly twice the nation's annual income. About a third of the approved lines were never constructed, and the ones that were built are largely the network Britain still uses.
A speculative belief detaches asset prices from fundamentals; the belief breaks and prices collapse.