RER A
Initially, the new RER D was meant to share with the RER A between Paris-Gare de Lyon and Châtelet-Les Halles. But, RATP, the company who runs the RER A, objected to such an operation as the number of passengers using the RER A was growing and required running extra trains on the RER A. It was decided that instead, each lines must have its own platforms, in which the RER A at the Gare de Lyon has its tracks at lower level of the underground station, with the future RER D on the upper level. The RER D tracks at the Gare de Lyon have four tracks and being above the RER A tracks, allowed "platform to platform" transfers vertically, a Japanese invention.