 Line 11 Châtelet - Mairie des Lilas was the first line to be converted, in 1956, chosen because of its steep grades.This was followed by Line 1 Château de Vincennes - Pont de Neuilly in 1964, and Line 4 Porte d'Orléans - Porte de Clignancourt in 1967, converted because they had the heaviest traffic load of all Paris Métro lines.Finally, Line 6 Charles de Gaulle - Étoile - Nation was converted in 1974 to reduce train noise on its many elevated sections.