Arriving in CDG terminal 1 midday from USA. How long should I allow to get through immigration etc. from the plane to the curb? Then taxi to St. Lazare for train to Bayeux. Trying to decide train reservation time.
Impossible to tell. Maybe a half hour, maybe more if the E gates are down. I would give myself at least 4 hrs.
Just wait until you get to Paris to buy the ticket since there is no guarantee that your flight is arriving on time also the trains on that route are regional commuter and do not require a reservation and don't sell out.
You can buy it on the SNCF ap after you pass through passport control as long as you have internet connection. https://www.sncf-connect.com/en-en/tools/mobile-app
Wait till you are on the ground. No way to predict the time -- should take about hour, more or less. IMO, it is not smart to booking on-going reservation after arrival. However, have done it with four and five hour windows --- But have been burned twice -- once when the flight was cancelled in the US and another time when the flight departure was delayed by about eight hours.