Railway signals. 19th-century illustration of five types of signals used on railways to direct locomotives and trains. The hatching indicates the colours: vertical, red; horizontal, yellow; diagonal, green. The signal at left is red. The one at centre is red and white. The one at centre right is yellow. The one at right is a red circle and a white and green square. Artwork from 'Chemins de Fer, Automobiles' (1911) by French civil engineer Max de Nansouty (1854-1913), part of the 'Les merveilles de la science' series of 1867-1891 by Louis Figuier.