Hi everyone,
I've got 3 pairs of train tickets to buy and I'm a bit confused about which ones I need to book in advance and which ones can wait until the actual day. I guess it's actually 4 tickets as our first one is CDG to Colmar, but I've already taken your advice and not booked that one since we don't know how our overseas travel will play out!

After our time in Alsace, we head to Provence (Bonnieux). We decided to do a one night stop over in Lucerne on the way. So I need to book Colmar to Lucerne. It looks like it's a TER train, and I believe those don't need advance bookings. Please correct me if I'm wrong. And just because they don't need advance booking, is there any advantage or disadvantage to doing just that?

Second one is after our one night stay in Lucerne, we'll be travelling to Avignon to pick up a rental car at Avignon TGV. I was on the My Swiss Alps forum and was advised to split this trip up into Lucerne to Geneva ( book on sbb.ch) and then Geneva to Avignon book on SNCF. Lucerne to Geneva I can't see what type of train it is, but if I try to book it says it's too early to book this. Finally Geveva to Avignon on SNCF and it has both a TER train and TGV, so for this one I'm guessing I do need to pre book and reserve seats at least on the TGV section.
I hope this doesn't sound confusing, but if someone could chime in and offer any advice or let me know if I'm going about this the right way.
Thank you all in advance!

Yah gotta it - TER and below no need to pre-book - same price generally at station - TGV trains yes at www.oui.sncf. For lots on trainscheck www.seat61.com; www.budgeteuropetravel.com; www.ricksteves.com.

CDGto Colmar will cost a lot more than a deepest discounted ticket - consider staying Paris near Gare du Lyon and perhaps save a bundle to tickets and be rested when you go. Or build in a lot of extra time for planes to be late and if have a few hours to kill try some kind of meal at the famed Train Blueu inside Gare du Lyon - see current thread about that.

you do not need to book trains within Switzerland nor the train Colmar to Basel SBB where you will connect to the Swiss Federal Railways. But if you plan to take the TGV Geneva to Avignon when you travel to Bonnieux then you DO need to book that in advance and the earlier, the cheaper.

There are direct - no changes - trains from Luzern to Geneva every hour on the hour (IR15). Journey takes 3 hours. There is one TGV per day Geneva main station to Avignon TGV which leaves around 0800 so you won't make that on the day you leave Luzern. You will have to take a TER train from Geneva to Lyon-Part Dieu and connect from there by TGV to Avignon TGV. These TER trains run relatively often but do check the timetables to be sure you have an efficient connection in both Geneva and Lyon. The TGV you have to book.

YOu can buy tickets in advance if you want, but there is no train where you "need" to book them in advance, if you don't want to. There isn't a single train that won't allow you to buy a ticket at the last minute. The only need would be if it were some really popular time/date when seats could get sold out. You can get discounts on prices, but you can see that on the website just be switching dates from far off to near. But that's to save money, it isn't a requirement.

You can't reserve seats on TERs, however, and prices don't change, so there isn't any real advantage to booking in advance.

AS for Geneve to Avignon, you'll save money booking in advance, but even today, you can buy a ticket on a direct TGV on that route for this Saturday. Tomorrow those trains are sold out (the direct ones, which aren't many), though. Plenty of tickets for those that have connections, though. This Sat it's 86 euro, but if you buy one for two months off, it's 53 euro. So you'd save about 30 euro booking far in advance.

Precisely - the TGVs sell out. Which is why one should buy TGV tickets in advance. They come with seat reservations. But there is another more important reason - the earlier you buy a TGV ticket the cheaper it will be. That doesn't apply to the TER etc trains and certainly not to Swiss trains unless you are buying a Saver Day Pass or a SuperSaver ticket...

In OP's other thread they mention that they will arrive August 30th at Paris CDG airport.
Which, AFAIK, is the end of the vacation month in France.
Here they say that they plan not to book tickets for that very first leg from CDG to Colmar in advance.

While I would agree that it is a bit of a gamble to know when your plane will actually land (with or without delays), I think that seats on those few good TGV connections from CDG to Strasbourg will be in high demand, especially on that weekend.
Fares for restricted tickets in 2nd class are already up to €77 on that day, only little difference already to restricted fares in 1st for €85.

To buy a ticket Luzern is perfectly possible - www.trainline.eu should sell these.

The reasoning could be the price, don't you think?

Pricing and the train timetable is the main reason for splitting these up. sncf is very expensive when searching for Lucerne to Avignon and the times aren't ideal either. When I do a search and split them, it works out much better. Lucerne to Geneva on sbb site and Geneva to Avignon looks similar on both sncf and sbb.

As for Paris to Colmar.....we arrive around 0600 and the train leaves at 0900. I was advised that three hours is the bare minimum to get through everything and make the train and that's with no delays or anything. There is another one at 1124 I could book. Five and a half hours after we're supposed to land. I think I'd be safe doing that, but I'm still a bit gun shy! There are still seats available right now and the prices are still the same as a couple of weeks ago.

Thanks again for all the input everyone. I've learned a lot from all of you!

If you are arriving by air at CDG and then taking a train to Colmar I suppose 3 hours is a reasonable margin. RER to Roissy - just over 30mns - then connect to TGV - allow maybe 20-30mns if you do not know the Gare du Nord OR if you have a lot of luggage. Getting off the plane, Immigration and Customs, finding the RER station - I would say at least an hour. So with your schedule you have 30mns for the plane to be late. I am not sure this is enough. I would leave more time...but then again hanging around the Gare du Nord is not all that nice.

If this works - fine. The SBB site gives an RER connection to the TGV via the Gare du Nord though.

I use the SBB site because it is comprehensive, reliable and easy to use and furthermore since I am Swiss it is a natural thing to do. It always gives the shortest connections as well...

No doubt it is the best site for Switzerland. If it is not listing the TGV from CDG it is obviously not the best site for France. Which is not surprising. And bahn.com is the best for Europe as a whole.