Caption:
Primrose Hill Tunnel near London, on the London and Birmingham Railway, opened in 1838. Engineer Robert Stephenson (1803-1859) Built through difficult London clay, the 3/4 mile tunnel cost £240,000. The portal cost £7,000. Picture shows elementary signally by bells, policeman with flag, and early rotating disc on a standard. From "The British Gazeteer" (London, 1852). Engraving. (Photo by: Universal History Archive/UIG via Getty Images)