We are taking several trains in France this July. My question, should I buy tickets in advance for
Paris- Bayeux round trip or just get them the day we arrive? What about Paris- Montreux, Switzerland?
Thanks in advance for your help.

Paris-Bayeux could have discounts if booking far enough in advance but the cheapest of those are often train-specific and non-changeable non-refundable. You can always get on those trains just by buying tickets day of travel. Trouble with buying tickets that are non-changeable with day trips is you never know when you will be ready to return. So leaves lots of time or just buy full-fare ticket for return that can be used on any train - "fully flexible" ticket.

Paris to Montreux discounted tickets can mean huge savings over walk-up fares. www.oui.sncf -French National Railways site for both. www.seat61.com has loads of great advice booking your own tickets online; general info trains - www.ricksteves.com and BETS-European Rail Experts. Swiss trains: www.swisstravelsystem.com; www.sbb.ch.

What are your plans in Switzerland - if several trains the Swiss Travel Pass could be a great deal

We are going to get the Swiss travel pass as we have a good amount of train travel there. Thanks for your reply. I hate to pay more for a ticket than I have to but there are downsides to non changeable tickets too. For some reason, the train portion of this trip has just befuddled me, even with the great sites you referenced.

Note that discounted tickets Paris-Montreux are better than say going to Basel or Geneva or Lausanne (all have TGV trains from Paris) and using Swiss T Pass to go to Montreux. Do not use your Swiss Pass that day.

<<You can always get on those trains just by buying tickets day of travel.>>

The problem with that is that Paris to Bayeux in July means a lot of people wanting to take that route, which means everyone who waits until the day of travel to buy tickets has to stand in (a usually long) line at the station. If you know what day and time you want to travel, it just takes a few minutes to buy tickets on line; then you don't have to get to the train station early to hope to buy a ticket, which is a waste of travel time IMO.

Any long-distance route is far better purchased in advance (like 3 months in advance for the best prices).

Just buy full-fare tickets ahead of time or buy them at any Paris train station day or so before to avoid potentially long waits in line.

Consider going first class to guarantee more empty seats if just showing up with full fare ticket - better yet when buying make sure you make the optional seat reservation.