Travel from Paris to Wengen (Switzerland)

Not possible. The best you can hope for is 3 changes, Basel, Interlaken Ost, and Lauterbrunnen. For one thing, at Interlaken Ost the tracks change to meter gauge from standard gauge, and the trains from Lauterbrunen are of a special design to handle the tight turns and steep grades to get up to Wengen. Trains run at least every 30 minutes, so if you miss a connection, just wait, there will be another along shortly.

I have seen maybe 1 direct train from Paris to Interlaken, but i don't see it for a random July day. Maybe only on weekends and/or during ski season.

I will be travelling on Sep 18 from Paris to Wengon.

3 change minimum. Best departures are at 10:23 am and 12:23 am for a 6 hour journey. It is only a 5 minute change at Basel, but it is very easy. You arrive on Track 9 and you just cross the platform to Track 10 and board the waiting train.
Interlaken Ost is a 7 minute change and you will have to use the pedestrian tunnel downstairs to change platforms. Take care to board carriages marked "Lauterbrunnen", as the train splits half way and the other half goes to Grindelwald. At Lauterbrunnen, you have 12 minutes, which is forever in Switzerland and you will see the connecting trains sitting there and station personnel instructing you to "Board any train!" That is because the tracks are so tight, that they cannot connect more than 2 or 3 carriages together. The solution is the 1st train departs, then 30 seconds later the 2nd train departs and follows right behind it.

As I said, all of these connections will have another train along in 30 minutes if you miss one. At Basel, the next train may require another connection in Bern, but you'll still get to Interlaken Ost.

The direct Paris-Interlaken TGV ran for a couple of years with limited timing but has been dropped, according to a railway rep earlier this year.

I am trying to book tickets from TGV website for Sep 18, 2018 from Paris to Wengen but website says "It is too early to book the requested journey. Please select an earlier travel date or set-up an email alert to be contacted when your journey is available."
When I will be able to book online through TGV?

Try this
https://www.trainline.eu/search/paris/wengen/2018-09-18-06:00

or use the Swiss Rail site
https://www.sbb.ch/en/home.html

The French rail site does not sell tickets to Wengen.

You have a couple of options.
You can book now as far as Basel at https://en.oui.sncf/en/tgv
I see nonrefundable tickets for 50 EUR per adult. You can buy the Swiss tickets later.

You can book now at www.sbb.ch/en
I see nonrefundable tickets for 132 CHF per adult.

Now, you have yourself and 2 kids. Anyone else? It is time to think if you will benefit by getting Swiss Half Fare Cards for the the adults costing 120 CHF for adults. If the adults have at least 240 CHF worth of tickets planned while in Switzerland, this will save money. Further, with that you can get a free Family Card and then the kids will travel free with you in Switzerland. The portion from Basel to Wengen is nominally 75.40 CHF. So if all the lifts you plan on taking while in the Berner Oberland, plus the trip as you exit Switzerland amounts to more than 165 CHF (very likely) you can benefit from the card. For instance, the 132 CHF ticket for Paris to Wengen is 96 CHF if you have a Half Fare Card. That is 50% off for the Swiss portion of the train ticket.

Will I have to buy Swiss Half Fare before booking my train tickets? Can you please provide the steps so that I could understand. I am not familiar about this Swiss Half Fare etc. Thanks

You do not need it when booking the train, but you must have it in your possession when the conductor checks your ticket, otherwise you are traveling on an invalid ticket.
You can buy it on-line and either print-at-home or downloaded to your smart phone with the SBB ap. You will also see a box to check if you want the free Family Card.
https://www.sbb.ch/ticketshop/b2c/adw.do?sprache=en&artikelnummer=11528

You could also buy it at Basel station if you decide to take a 30 to 60 minute break on the way to Wengen.

I was trying to book train tickets for Sep 18 with return Sep22 but not able to book at the following website. I want to book one ticket from Paris to Wengen (Via Basel SBB - Interlaken Ost - Lauterbrunnen ).

https://www.raileurope.ca
"It is too early to book the requested journey. Please select an earlier travel date or set-up an email alert to be contacted when your journey is available."

I was just able to get round trip Paris to Basel for 1 adult and 2 kids 8 & 10 for 239.30 EUR. That is the direct 10:23 departure, and the return at 13:21 with a connection in Strasbourg. Like I said, pick up a Half Fare Card and the free Family Card at Basel station on the way, so you travel half price and the kids travel free with you in Switzerland. With the Half Fare Card, that is 75.40 CHF round trip to Wengen.

Hi, I will buy Half fare Card and I have 2 adult and 2 kids ( 10 and 8 yr) and want to book ticket from Paris to Wengen with minimum stop time with the connected train. If I book from Paris to Basel then how I can book from basel to Wengen. I think the waiting time will be little more if I book train from Basel to Wengen. I want to save time and want to reach in 5-6 hrs in Wengen from Paris. Is there way to book one ticket with the three stop with minimum waiting period to pick the next connecting train reach in 5-6 hrs in Wengen from Paris. Please also send me the link so that i can book ticket online.

I found this at www.trainline.eu for 383 EUR total, 2 adults, 2 children 8 & 10.
Sept 18 Paris Gare de Lyon depart 10:23, arrive Basel SBB 13:26
Sept 22 Basel SBB depart 13:21 to Strasbourg then a change to TGV to Paris Gare de l'Est arriving 13:35. the TGV tickets are 1st class, better deal.

Book Swiss tickets at www.sbb.ch/en. You can buy the 2 Half Fare Cards at 120 CHF each plus the free Family Card. It is too early to book Swiss tickets now, they only book 30 days out. When the time comes, book a departure from Basel SBB after your scheduled arrival time of 13:26. Don't worry if you miss it, your ticket is good for the rest of the day to get you to Wengen. Book the return to get you to Basel SBB station well before 13:21. The tickets from Basel to Paris are nonrefundable tickets and you must make that train, or have to buy new full fare tickets. Leave an extra half hour in case something goes wrong.

You might want to start now by downloading the Swiss Rail app. It will make buying tickets easier and you can even have the tickets loaded on your smart phone.

Lastly, you can ask questions to agents of SBB at the website contact link at the bottom of the web page. They will be happy to answer specific questions, usually replying the next business day.

Hi,
How you found the price for 383 EUR total for 2 adult and 2 children 8 and 10. I tried this link (www.trainline.eu) but not able to book and asking "Loyalty and Discound card" number for adult.

I also tried to book tickets from www.sbb.ch/en but the price is 898 Swiss franc with the two half fare card. Is not too much?

I just went through it again. The price has gone up a little bit, not huge. Now 442.80 EUR. Prices will continue to rise as you get closer to your travel date, so you should buy now. Loyalty and discount cards as blank, as you have none (I assume).

I just checked. Basel SBB to Wengen is 37.70 CHF per adult with a Half Fare Card. Kids will be no charge with the free Family Card you get when you request with the order of the Half Fare Cards.

SBB does not sell domestic tickets until 30 days before the travel date. You can buy the return tickets when you are in Wengen.

I just went through it again with (www.trainline.eu) but still not able to go in next step. How I can leave "Loyalty and discount card" as blank.

Don't know what to tell you. I just got to the payment page with 4 fake names. Did you put in the kids ages, then click OK?

Now I was able to book but the price is much more than 442.80 EUR. I see the following information for 2 adult and 2 kids. Am I doing correctly?

Return journey from Paris to Wengen
for 2 adults and 2 youths,
10:23 – 16:49 on Tuesday 18 September,
06:42 – 17:35 on Saturday 22 September.

€765.40

I am afraid you did not listen and booked tickets all the way to Wengen. Oh well, enjoy Wengen. You are aware that the last train you booked from Strasbourg to Paris is a Ouigo train, a special no-frills train that operates like Ryanair, you have to pay extra for extra luggage. Have some bottles of water with you for the kids, as there is none on the train.
https://www.sncf.com/sncv1/en/trains/ouigo

PS, You really did not overpay, at least that much. With 2 Half Fare Cards you would have paid about equivalent of 336 EUR, so you would have paid equivalent of 778 EUR. Its is only that you would have benefited from the Half Fare Cards and the Family Card in Wengen. Have fun.

For anyone else looking to make a trip like this, we were able to go Paris > Lausanne on SNCF, bought Half Fare Cards and half-price tickets for Lausanne > Bern > Interlaken > Lauterbrennen > .Mürren in the Lausanne train station. So if the trains via Basel are full or too costly, there are other options.