I am planning on using RER C to get to places on both ends of the line. One time to and from Versailles Chateau and another time to and from Dourdan. The time frame is from Sept 11-14. The closest station to our apt. is musée d’Orsay.
I have used multiple apps for plugging in itineraries and I get different results. I know this is all due to the summer railway construction and recently I have at least been able to get some options that these trips may be doable.
The apps I have used are Île de France mobilities, RATP.fr, Citymapper transillien website, and SNCF connect. With this major rail network construction is there one app you find more updated and accurate? Going from one app to another one I will see the itinerary is scheduled and gives times and then the next one will list that itinerary not available. I do understand the difference between which “companies” run the various methods of public transportation. Do you think if I talked to someone at Île de France mobilities ticket office at CDG they would give accurate info for dates and times we want to travel on RER C?
Any Parisiens that actually have a present onsite, on the ground experience with what has been going on with the construction?
I am confused if you can use the RER at all if it says service interrupted between 2 different stations. Does that mean the train just doesn’t stop at those stations or can you literally not got from one end to the other because the train stops and starts up again at a different station.
Really was hoping could use public transport but wondering if I should be prebooking taxi especially to Versailles if it is such a major interruption that there may be minimal availability.
Thanks for any input as everyone has always made trip planning a little less stressful!

The whole point of the RER is to not have to worry about schedules other than the first and last trains of the day. On the RER C, you have a train about every 5 to 10 minutes or so. Since not every train goes to every destination, you just have to keep an eye on the electronic route signs before boarding.

During the period (in July and August) when there are service interruptions due to track work, you should know that you can also take trains to Versailles from Saint Lazare or Montparnasse.

I realize that the RER C is frequent but what I was using the apps for is how to get from point A to Point B at specific times times using buses, metros and RER C due to the fact that most museums require timed entries that have very little leeway if not present at booked timed. It seemed like what are call itineraries were not consistent between apps. If the service interruptions are only July and August that would be wonderful for me! There seem to be many listed for September and beyond! That is why I was hoping for a response from someone who is living the disruptions. I guess I was looking at short direct routes and going to and from Montparnasse seemed to be out of the way- what line is that one?
Thanks!

kerouac was referring to regular SNCF trains from Montparnasse, not the RER, it doesn't stop there.

They do have a lot of apps, I guess several cover the RER. I would presume RATP is the only one that covers metro, but don't know as I don't use apps. I just use the maps to figure out where to go, but I don't ever try to time things to the second that it matters that much to me. I guess if I were doing a special trip outside Paris I might check to make sure some station wasn't closed I intended to use or something.

I just use the website, ratp.fr which tells you specifically all the works going on and closures etc (in the traffic section), It also as an itinerary planner so presume that's the same as the app, but don't know. There are no issues with Musee d'Orsay going to/from Versailles in September. It looks like Dourdan stop won't have service some dates but not until 9 pm or later, that's all. IF you are going daytime, no problem. The ratp website tells you specifically what things mean, usually just a station is closed but if there i no service between two points, it tells you if a bus is running to service those particular stations, or what.

I am not sure what confusion you are finding, I just checked RATP versus the Mobilites site for Musee d'Orsay to Versailles Rive Gauche on 9/11 and they show exactly the same thing (for today, also). Just a direct trip that takes 30-40 minutes or so. If you are worrying about estimates of a few minutes difference in travel time, I think you just have to let that issue go and not plan things that are that tight. They are just estimates, after all.

I would not even consider trying to contact them by phone to get personal service on exact times to the minute for some trip you are planning. I would mainly use the RATP app myself. I don't know what itinerary you are finding not available for the two examples you are concerned about. I checked 9/13 for to/from Dourdan and both RATP and Mobilites agreed just a direct trip, takes about 1:10 hrs.

Thank you SO much! Yes updates to things in the past few days seem to show less interruptions for September for the RER C. Since I happen to be a planner(which is why I love fodorites input) I was probably looking too far ahead which made it confusing as to what would be available!
When it comes to time, a few minutes difference is not what I am looking for but since we have early tickets to Versailles plan to be there plenty early ( therefore flexibility). Since our tickets are first entrance time don’t want to plan more than an hour prior to start time and then find out last minute at RER station that I misinterpreted itinerary/station due to construction.
Again thank you so much for understanding what information I was trying to verify! I feel so much better that someone is reading it the same as I am for Dourdan meaning disruptions will be later in the day.