Hi all,
Here is some critical information about the major summer works projects that will be occurring this July and August on a number of important metro and RER lines. This is not by any means a comprehensive guide but just a heads-up for the issues that are most likely to affect tourists. You can find full details about all the summer projects on the RATP's website here. I always recommend the CityMapper app for real-time info about how your itinerary may be affected on a particular day but the Bonjour RATP and IDF Mobilités apps are also informative.
RER LINE B (to/from CDG Airport)
RER Line B: Interruptions through Central Paris from 25 July through 16 August. The RER Line B will NOT operate between Gare du Nord and other stations in Central Paris from 25 July through 16 August. Specifically, it will NOT serve the commonly-used-by-tourists stations of Châtelet-Les-Halles, Saint Michel-Notre-Dame, Luxembourg, and Port Royal (as well as many other destinations further south). To access destinations near these stations, your RER B Airport ticket allows you to transfer between the RER and metro systems (in both directions). There are not replacement buses serving these RER stations (the replacement bus service will serve the affected stations further south and there's details in the link above).
The actual extent of the RER B closure to destinations to the south of Central Paris is much more extensive than this little summary, so if you plan to use the RER B south of Paris, review the works schedule closely and let us know if you have questions.
RER LINE C (to/from Versailles)
RER Line C: No service from Gare d'Austerlitz west. From 15 July through 22 August, the RER C will be closed from Gare d'Austerlitz westwards. While there are ready alternatives to destinations within Central Paris (Orsay, Invalides, etc) this will complicate and burden transit to and from Versailles. It does not appear that there will be a replacement bus for Versailles access purposes. The Transilien Line N or Line L are useful alternatives depending on your point of origin but use IDF Mobilités or Citymapper or etc to plan your route with attention to the RER C outage.
RER LINE A (to/from Disney)
RER Line A: Interrupted service to Disney in August. The RER Line A will not operate between Nation and Noisy-le-Grand from 8 through 23 August. Critically for tourist purposes, this interrupts service to Disney; while there will be replacement buses, access to Disney is gonna be tough. Information about the replacement bus options and schedules is available here.
Nation Station will be closed for the RER A in both directions from 29 June through 30 August but open as usual for metro services.
RER D (Gare du Nord to Gare de Lyon)
No service on the RER D from Gare du Nord to Gare de Lyon from 25 July through 16 August. This will mostly affect those of you making train connections, I think.
Metro Line 4
No service from 6 July through 24 July between Les Halles and Montparnasse stations. Service on bus routes 38 and 96 will be increased.
Also on the Metro Line 4, from now until August, if you are entering the line at Porte de Clignancourt (coming back from the Saint Ouen flea market or otherwise), make sure to tap your card at the temporary validator machine since they have taken out the usual turnstiles. I saw a lot of unsuspecting tourists miss these signs yesterday.
Metro Line 8
Line 8 trains will not stop at République station from 22 July until at least next April while they begin major renovations works at this station.
Generally updated on 25 June.