Until recently the only way possible to get a Seat Reservation for Paris St Lazare to Cherbourg was at an SNCF Ticket Machine in France, which is far from convenient, especially if you live on another continent.
It is now possible to make online Seat Reservations for the Paris St Lazare to Cherbourg Rail line on Raileurope.com
As of this morning January 23, 2024, you can make Seat Reservations upto March 24, 2024.
As far as I know, at this time, Raileurope.com is the only way possible to make online Seat Reservations for this Rail Line.
Hope this is of some help to others going to Normandy. It certainly lifted a lot of weight off my shoulders.
The idea of spending thousands of dollars, then traveling thousands of miles and then having to wait until just a few minutes before boarding a train before I could even know if I could get on a very much needed train is unnerving.
Ken