It doesn’t double the capacity. It’s more a 30% increase. And, double-deckers are actually old. They date from the 19 century. In 1855, the Chemin de fer de l’Ouest introduced double-deckers on suburban lines of Paris.

fr.wikipedia.org/wiki/Voiture_Ouest_à_impériale

Modern double-deckers date from the 1930s with the Voiture à étage Etat:

fr.wikipedia.org/wiki/Voiture_à_étage_État