Detail from Harry Beck’s 1951 Paris Metro map design (which was rejected by the city’s transport authorities). Source: London Transport Museum
Harry Beck: The Paris Connection:

The Royal Mail recently commem­orated one of the UK’s greatest works of visual infor­mation design when Harry Beck’s London Underground diagram was included for the first time on a British postage stamp writes Mark Ovenden. The impor­tance of Beck’s rectilinear, topologic 1933 diagram is widely recognised and praised by graphic designers. Many wonder why Beck never extended his ideas outside London. The answer is, he did – to the nearest major subway network to London: Paris.