Upvote:2
As others have said, it's the same procedure to go back the other way, as covered by the previous answer.
The area which services SNCF trains is slightly different to the area of the metro (on both ends) so you will need to follow the signage. However it is fairly clearly signposted.
You would need to take Metro Line 4 in the direction of Mairie de Montrouge. You would exit at Montparnasse - Bienvenue, and you would follow the signs toward the SNCF area if that's where you are going.
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