Finding Train Platform

There are big screens hanging from the ceiling with all the departure trains on them. Sometimes they just have the end station so maybe Caen was listed as an intermediate stop on the scroll underneath the train. The trains are listed by time and too far ahead and they will have the train but not the track (voie) number.

In Montparnasse, walk out into the area where the heads of the tracks are and you will see people just standing in certain areas. They are monitoring the reader boards. All of a sudden you will see a bunch of people peel off and head in one direction because their track numbers have been posted.

editing to add: Here is a youtube video filmed at Gare Montparnasse. Both at the beginning (static picture but I see it does not always come up), at about 3:21 and at about 6:15 the person walks toward big blue and green screens. The blue screens are for departures, green screens for arrivals. I think the white circles on the floor were put in for social distancing while people waited for track numbers to come up.

https://www.youtube.com/watch?v=aGIpIH-5m2E

If you arrived at the station early, the platform number was probably not listed on the monitors as it had not been assigned a platform yet (or stations where staff control entry and aren't ready yet). This is why there are usually large groups of people standing under the monitors waiting for the platform number to show. It can also be confusing because the station listed as the final destination might not be your destination, but they do list platform numbers at St. Lazare. I think that train is a TER train is probably why your seats were occupied. Not that it happened here, but sometimes it can be confusing which car is which class (same car number with same seat numbers).

Ouigo is simpler because you have to arrive early or the boarding process and there is one ouigo train to bordeaux at that time. So there won't be issues.

Hopefully you don't get sick. The prevalence is rather low at the moment. When I had Omicron (in France), I tested negative four days after symptom onset. If you do get sick, buy some of the home tests so you can determine when you're negative without having to go to the pharmacy...

There are multiple trains using the same platforms, often only minutes apart. So if they post the platform number too soon (while the other train is still there), you might get on the wrong train. It doesn't take long to board a train, so dont panic. If you're in a pinch, just get on any car and you can find your car when you're aboard.

Thank you all for the advice. Tomorrow we have an hour and a half to get from Gare Lazare to Gare Montparnasse. We intend to take the metro. The Montparnasse metro station is very large and the Gare is at the other end, so I hope we don't have any issues. I will report back about how it went.

Take metro line 13 from Gare Saint Lazare to Gare Montparnasse. It is quicker than line 12, and it gets you much closer to the train station at Montparnasse.

Thank you very much balso, I will do that!