The RER plan initially included one east–west line and two north–south lines. RATP bought two unprofitable SNCF lines—the Ligne de Saint-Germain (westbound) and the Ligne de Vincennes (eastbound) with the intention of joining them and to serve multiple districts of central Paris with new underground stations. The new line created by this merger became Line A. The Ligne de Sceaux, which served the southern suburbs and was bought by the CMP in the 1930s, would be extended north to merge with a line of the SNCF and reach the new Charles de Gaulle Airport in Roissy. This became Line B. These new lines were inaugurated in 1977 and their wild success outperformed all the most optimistic forecasts to the extent that line A is the most used urban rail line in the world with nearly 300 million journeys a year.