tramways that ran on normal streets. This was done with the law of 31 July 1913, designating them both under the name VFIL and establishing a new, more-logical classification distinguishing railways and urban tramways. The Inter-war period saw new laws (of 1 October 1926 and 17 April 1927, for example) which, with their measures of decentralisation and removal of red tape, tried to ease the financial difficulties of companies already closing their lines and often replacing them with road transport. Though the VFILs made up a baby boom, their lives were brief; only two or three generations will ever have seen them in use. In 1928 the various networks were at their largest, . In the Second World War many lines closed, victims of both the road and their own slowness. Not long after the end of the war, from the early 1950s, the survivors fell one by one. But a few still survive, sometimes as heritage railways such as the Chemin de Fer de la baie de Somme. The development of secondary railways happened at the same time throughout Europe. In Belgium, the SNCV created infrastructure and rolling stock to respond to the same need, but they evolved differently for many different reasons (construction by a single national body, the higher population density, a greater number of connections, partial electrification, and so on) and their development culminated around 1950. Minimal cost On the secondary railways, everything was designed to save money, which did not necessarily entail poor workmanship or mediocre service. Lines were generally narrow gauge, varying from (Calvados network) to as was more common. There were instances where standard gauge of was adopted, often to facilitate the transfer of through traffic to main line railways, such as on the Montérolier-Buchy - Saint-Saëns line in Seine-Inférieure. To keep construction costs down, routes followed the terrain as much as possible, with gradients as steep as 1:22.22 (4.5%) to 1:20 (5%) compared to no more than 1:40 (2.5%) on more traditional lines, with the exception of some mountain railways. Curves could have radii of less than . The rail tracks used were very light; generally Vignoles rail, with mass being depending on the distance. In many cases, lines were laid over road shoulders, which reduced the need to buy land and, above all, limited the need for new bridges and tunnels. But these measures severely limited the maximum operational speed, generally