RER C Train From Paris to Versailles

from where in Paris?

The guidebook does not, and cannot predict when they decide to schedule maintenance shut downs of rail lines. I take it you are looking at going to Versailles on a weekend. The RER C line in Paris is shut down on the week ends as they work on the tracks. That line is normally about a 35 minutes ride from Paris, depending on what station you board. On weekends, for the time being, you have to take other rail lines to get to Versailles. see this current thread.
https://community.ricksteves.com/travel-forum/france/rer-c-train-and-versailles

Actually, looks like it will be shut down even week days in the near future. Use this site to plan on how to get there. https://www.ratp.fr/en/itineraires

Just a quick look shows 41 minutes from Invalides by taking the No 13 Metro to Gare Montparnasse and then the "N" train to Versailles Chantiers, then a bus to Versailles Rive Gauche.

We would be traveling on Tuesday 7/26 from St. Michel station.

RATP suggests taking the No 4 Metro to Gare Montparnasse, then the "N" train to Versailles Chantiers, then walking from there. Takes about 1 hour.

There’s another thread about this today with a bunch of responses

https://community.ricksteves.com/travel-forum/france/rer-c-train-and-versailles

As Sam wrote, take the metro to Gare Montparnasse and then a Line N train to Versailles Chantiers. It is about a 15-20 minute walk from there to the Palace. I'm providing two links to help with this route. montparnasse-to-versailles-chantiers
For this link scroll down to read about transferring from metro. metro transfer

When we visited we had the early morning private apt. tour arranged. By luck we were ready early and decided to go ahead and amble around Versailles until time to enter. We got onto the RER C at St. Michel, but two stops later it halted and the few people on it started getting off. Another passenger told us to get off - work on the line and not going further. Unexpectedly we had to grab the metro to Gare Montparnasse. Once there it seemed like we walked forever to get from the metro into the train station proper, no other people in sight in the dimly lit halls. Sorta spooky and disorienting. Finally we saw a sign for ligne N and followed that. After that the trip was uneventful and we arrived at Chantiers with plenty of time to walk to the palace.
Upon leaving that afternoon we walked back to Chantiers planning to return the same route. Checking with workers that it was the train to Montparnasse, we boarded the train back to Paris. It was a packed train and we were relieved to find places to sit. We rode along for awhile and then looking out the window I saw Tour Montparnasse, far, far in the distance. Uh oh.
Well, we were on a train to Paris, just not the one we thought we were on. Turned out we were on the line U train, not the line N train. Hahaha. We exited when we arrived at La Defense and then caught the metro 1 back to Hotel de Ville and it turned out to be an easier return walking wise than had we caught the line N train.
Have fun and be prepared to adapt if necessary.

thank you Nance for those links! super helpful and I had not seen them before.

Thanks All! This is super helpful. Now we are trying to decide whether we just stay in Paris since we are traveling with our 6-year-old and only have three days.

I just got back from Paris and the 15th of July was the first day the RER C route was closed which was the day we were to visit the Versailles so my wife and I had to rejig our plans taking the L Transilien train from St Lazare to Versailles Rive Droit then a quick bus ride to the gates.

I would not take a 6 year old to Versailles especially with so little time in Paris. It is really unpleasant especially in summer for a child who will be seeing nothing but butts jammed into the palace scrum.

With a 6-year-old kid in tow - I'd be including a pretty good chunk of the middle or last day in Paris at the Jardin du Luxembourg - that would include a nicely assembled picnic lunch.