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The CMP immediately proceeded to upgrade its new property, electrifying it with an overhead system down to Saint Rémy-les-Chevreuse in 1938.

The outlying section to Limours, where traffic was minimal, was never electrified and closed to passengers on May 15, 1939.

From this time on it was generally considered as a metro line, even though it ran on different gauge and with different rolling stock.

Maps of the metro network show it along with the numbered lines with an ' S '.