Train from Paris to Lucerne in June 2024

Have you looked on the SBB website? Thats the Swiss rail line. DB doesn't sell tickets for trains outside Germany. Omio is is a 3rd party line - deal with the actual train company whenever possible.

It’s too soon to book on France’s SNCF website but I see a fare two months out taking 4h 45m with one transfer in Basel: https://www.sncf-connect.com/en-en/. Switzerland’s SBB website is https://www.sbb.ch/en/timetable.html.

Yes, I also looked on SBB's website, and got this message:

Your journey could not be booked. The train is already fully booked or has been cancelled due to engineering work.
You have the following options:
We recommend selecting another connection.
For connections with changes: It may be possible to complete the purchase if you split the booking into the individual legs.

I got this same message after choosing other options. Strange! Maybe it is too early to book. but I'll try the links sent by the other person who replied. Thanks!

SBB should really work on their error messages. That message you got is getting famous... (And it is completely wrong).

I will repeat it again: Trains do not sell out months in advance. If they did, they would be useless.

You can book this ticket on multiple sites (even on DB!) but not yet. It is just to early. And an additional problem is that most booking sites have not been designed with foreign tourists with seat anxiety in mind.

When you try to book what happens is that the system first looks up the timetable in the MERITS system. And here you run in the first issue when booking to far in advance: Weird routings. If for example DB and SBB have already loaded all trains till december 2024, but SNCF only till early june what happens when you try to look up trip you often get proposed long detoure. Eg if you look up Zurich to Milan in october you get send via Austria, because SBB, and OBB have their trains in the systmem already, but Trenitalia not..

Then once it has found your train the system looks if there are trains with compulsory reservation involved. And if that is the case it tries to temporarly reserve a seat for you. The problem is that the system some railways use (eg. RESARAIL) will only answer with "here is your reservation: car 25, seat 61" or "reservation not possible". It does not communicate why reservation is impossible. Hence for example SBB responds with a generic message that excels in being unhelpful.

And all this because the peopel making those systems are Europeans, and the thought of booking train tickets for June already in Februari does not cross their mind. (I am going to Italy in April, and haven't started booking trains yet either...)

So here is what you should do:

• Preferably book from the railway where your trip starts. That would be SNCF in this case. The only snag here is that SNCF cannot for some reason produce e-tickets for Swiss Domestic trains (in spite of the interface to do so being open to anyone), so you would need to pick up the tickets at a station in France. But since you are starting your trip at a station in France this is not an issue. (This is a problem also with some tickets from Italy and Austria, which is why "book from the railway where you start" is a good base). If you want e-tickets in hand book from SBB or Trainline (or in a pinch even RailEurope)
• Look at times and prices tomorrow, and a month from now. That shows you a) what service level to expect b) what the logical route is and c) how much cheaper tickets are when booked in advance.

That should give you an idea of what is available. If you then try to book your actual travel date, and what you see does not make sense you are just to early.

Regarding Omio: There are some agents that indeed will take your money, sit on it till bookings open, and then book the ticket for you when it does. You often end up paying a lot more that way. I would not bother. Just make a not in your calendar somewhere in April with "buy ticket Paris Luzern on www.sbb.ch" and let it rest.

I know a bit more now: Currently it is not possible to book Switzerland - Paris trains beyond May 24. Ticket sales for dates beyond that will start in early March.
Have a look at this page:

https://www.tgv-lyria.com/fr/en/booking/book-a-train/lyria-2020/sales-opening

The reason is that, because of the Olympic Games in Paris the whole summer schedule is still up in the air. It is quite probable that TGVs from Switzerland will be routed to Paris Est in stead of Gare de Lyon, to free up capacity in the latter.

So you will have to wait till March at least to book this trip.