My wife and I are planning a trip to Switzerland (my selection) coupled with a trip to Spain (her selection) in late July to mid-August. We will use public transportation in Europe. We have visited Switzerland once before, hiking around Arolla. My first thought is to fly into Zurich, take the train immediately to the Lauterbrunnen area for a 5-6 day stay in Murren or Wengen to hike in the mountains, and then take the train from there to Spain. After 10 or so days in Spain we would fly home from Madrid. We would enjoy visiting some of France, but my wife has spent the entire Covid period learning Spanish via the Internet and this will be her opportunity to use what she has learned. Nevertheless, an overnight or two in France could be accommodated. We're seniors, but enjoy walking/hiking/bicycling, learning about the local people and their culture, and spending multiple days in the same place. We visited Girona, Barcelona, and the Costa Brava area a few years ago, but nowhere else in Spain. Is there a particularly scenic French rail route between Switzerland and Spain? Is there a rail route through the Pyrenees that is particularly scenic? Should we spend some days in the Pyrenees? Given that the interior of Spain will likely be hot, is it unwise to head straight to Madrid and then use the RS suggested itinerary as a guide? Will the coastal areas be crowded? I haven't looked into rail passes yet, but plan to.
Madrid is often brutally hot in August, and so are Toledo, Segovia, etc. Would you rather enjoy a repeat visit to Catalonia, which should be more bearable?
Northern Spain is also an option, but for that I would just fly between Zürich/Geneva and somewhere in northern Spain such as Bilbao or Santiago, to visit either the Basque Country or Galicia.
As for your original question: no, there aren't any scenic rail routes between Switzerland and Spain, unfortunately.
The meter-gauge Yellow Train runs through the Pyrenees from Villefranche-de-Conflent (a fortified town) to Latour de Carol. Although popular with tourists, the train is an SNCF operation, so the cost is reasonable. There's some very nice scenery on the way, though I guess not as exceptional overall as the best routes in Switzerland. You can get to Villefranche by regional rail from Perpignan. Perpignan is not one of France's most gorgeous cities, but it's interesting and blessedly non-touristy. At the other end of the trip, one of the last stops is at the small town of Bourg-Madam, which is virtually on the Spanish border. About a 40-minute walk (or I'm sure a taxi could be arranged in advance) from Bourg-Madam is the very pretty (and rather touristy) larger Spanish hill town of Puigcerda (Poo cher DAH). Puigcerda is in the Cerdanya Valley, which has some very historic little towns/villages accessible by bus. However, be warned that the Cerdanya Valley can be shockingly hot in the summer for a place so very close to the Pyrenees. Puigcerda and La Seu d'Urgell are attractive places worth visiting. Both have bus service to/from Barcelona, and I think Puigcerda also has connections to Girona. However, I was stood up by a bus on Sunday in a little town on that route, so I'd not totally trust the Puigcerda-Ripoll-Girona buses to run reliably. In any case, I wouldn't recommend a lot of time in that area in the middle of the summer. There are better options, meteorologically speaking.
As long as you stay relatively close to the coast, the whole stretch of northern Spain from the Pyrenees to Galicia is highly likely to have moderate temperatures even in mid-summer. The rare high temperature seems to show up for just a few hours in the middle of one day rather than for four or five days at a time. I haven't actually seen much of the Spanish Pyrenees at all, so I cannot speak from experience about that area. I found the most dramatic scenery in the Picos de Europa area, the Basque Country and Galicia (in that order).
There are lots of interesting cities, towns and villages in that part of Spain. I spent just under six weeks there in 2016 and didn't have time for all the places on my list. (I didn't have a car.) Among my favorites were the Picos de Europa (seen on a one-day bus tour from Santander), Bilbao, Vitoria-Gasteiz, Hondarribia, Burgos (warmer than most other places listed and with many pilgrims), Olite (warmer than Burgos, not quite as bad as Madrid), Oviedo (it helps here if you like really, really old churches), Leon (weather similar to Burgos), Betanzos, A Coruna, Pontevedra and Santiago de Compostela (more pilgrims). San Sebastian, Santillana del Mar and Pamplona are very touristy places. I thought the first two were still worth seeing (and of course there's the food scene in San Sebastian, plus the lovely beach); Pamplona I feel is skippable.
Most of the places I visited didn't have a lot of tourists, especially not foreign tourists.
There are active-sport opportunities in the Picos de Europa area. I thought the old stone town of Potes was really charming, but it is not undiscovered. There's a cable car at Fuente De; there was patchy snow on the ground at the top when I visited the area on July 7. There must be great hiking in the Pyrenees as well.
My earlier response did forget about the Train Jaune!
Not terribly convenient to include it on a long journey though.
I believe the Man in Seat 61 has recommended train travel agents in Europe, you might reach out to some.
Although acraven's post is pretty darned amazing.
Thank you so much for these suggestions. I think we will head from Switzerland to Spain's northern coastal area, visit Bilbao and some towns farther west, and try to spend a few days exploring Picos de Europa National Park, perhaps using Potes as a base if public transportation will allow us to do that. The "Man in Seat 61" website looks like it will be a great help for our travel from Switzerland to the San Sebastian area (e.g., using TGV trains, we'll need to transfer between train stations in Paris) and probably in Spain as well--many thanks for that website tip.
days exploring Picos de Europa National Park
Good idea, you basically split the difference, Alpine beauty but the locals speak Spanish, that's the Picos de Europa! FYI the Asturian accent may be difficult for non-native speakers to understand, but will definitely make for an interesting challenge lol. Not to mention the native Asturian language, which has a distinct grammar and dictionary to Spanish.
Enjoy the linguistic milieu we call Northern Spain! :)