How do I get to Mount San Michele from Paris by train?
You take the train from the Montparnasse station to Pontorson, then take a taxi from there to Mont-Saint-Michel. There are trains that run direct to Pontorson and other trains that change in Rennes. You can get schedules and fares on the SNCF website.
There is a shuttle bus from Pontorson to MSM.
https://www.bienvenueaumontsaintmichel.com/en/preparing-your-visit/arriving-by-train
The bus schedule is here: https://www.bienvenueaumontsaintmichel.com/file/medias/doc-69-fiche-poteau-ligne-pontorson-le-mont-28032022-30062022.pdf
It appears the schedule may be adjusted after June 30.
In addition, when the direct train from Paris to Pontorson does not run (it is summer only), you can book a ticket from Paris to Mont Saint Michel by train to Villedieu les Poêles then 45 min bus to the Mont. This combo is sold on the SNCF Connect website. It is a little bit slower than the Rennes route, but cheaper and can be more convenient if you are OK with the early departure.
Thanks all for the help, it is perfect. I have booked the tickets, very easy to do.