The population boomed from 1950 to 1980. Car ownership became more common and suburbs grew further from the centre of Paris. The main railway stations, termini of the suburban rail lines, were overcrowded during rush hour. The short distance between metro stations slowed the network and made it unprofitable to build extensions. The solution in the 1960s was to revive a project abandoned at the end of the 19th century: joining suburban lines to new underground portions in the city centre as the Réseau Express Régional (regional express network; RER).