Renfe AVE with connections can't select seat? Not a fan of riding backwards.

Your last concern is the reality. While you can select the seat in a wagon, there is no way of knowing which way the wagon will go.

If your travel companion is willing to ride backwards you could reserve a set of seats facing each other, guaranteeing one facing each way.

Edit to add: This is a train journey against which flying would be an attractive, more pratical alternative.

Yeah, we know about the flight. We always take a train when we visit places so we thought that'd be it. Is the trip interesting? Otherwise, the flight may be a better alternative.

Maybe you’ll get lucky and the train won’t be full so you could move to a seat facing forward.

I get it—I'm a big fan of rail travel. But this ride is not particularly picturesque—mostly arid flatlands with olive (I think they were olives) groves.

Spain's a country with approximately a gazillion worthwhile towns and cities, but it has a lot of dull scenery in the middle.

I usually have no problem and rode a SNCF train backwards to my home in the south of France from CDG as recently as two weeks ago. In fact, I've taken a real rocky transatlantic with no problem. But as I write this, I'm in a Renfe, riding backwards and find myself having to take action to stay well. This train rides lower, has larger windows than French TGVs, and it's difficult to fix on distant horizons without the closer scenery throwing off my balance. It's easier to fix your eyes on distant mountains; flatlands are difficult.
Both 2 weeks ago and today we're in first, seats were assigned, no choice. Anyway, my choices always end up backwards no matter. I have never had a problem.

The NYTimes had an article on causes and avoiding motion sickness this past week. Keep your eyes fixed on something stable, or close your eyes and rest. I often watch movies on an iPad. Bon voyage and Adios.

I used trainline.com to buy our tickets from Madrid to Barcelona. It did have the ability to select facing direction of travel and quiet car. Renfe's site I found very confusing though it was useful in narrowing down which train departure time was cheapest.

Thanks for the replies.

I got a bit sick riding Portugal's Alfa Pendular and I was facing forwards. I think because of the tilting they have to go faster. In fact, I just read about it in the wiki page that I just visited to remember its name: 'the jostle and sway compensating for track irregularities may cause "travel sickness" in those susceptible.'