Hello! We land in Madrid Barajas at Terminal 4 at 14:15 then travel to Seville. I don't know how long does it take to go through customs and get to the Atocha station from the airport, Looking for an advise on choosing a reasonable train departure time that we can make, I see departures at 16:00, 16:30, 17:00, 18:00, 19:00. Also, I see "semi-flexible" choice for $8.50 per ticket, would you recommend to pay extra in case flight is delayed? Thanks in advance.

If you must buy in advance by all means get something flexible. The time from landing, taxiing to your gate, deplaning, clearing immigration, collecting your bags and then getting to Atocha is subject to so many variables - delayed arrival, long line in immigration and baggage claim, etc. Especially with the chaos that is besetting so many European airports these days, I would just wait until you are actually boots on the ground ready to get into your taxi to Atocha to buy your ticket. The taxi ride is about 20-30 minutes or so, you could purchase your ticket on your smartphone from the taxi. Or even just buy it at Atocha.

Where are you purchasing your ticket?
I ask because RENFE only charges in Euros.
There is no details on how much notice you have to give to be able to change. I doubt if you can change if you miss the train.
https://www.renfe.com/es/en/viajar/p...tancia/premium
There is also security for high speed trains. Some allowance in your schedule for this too.

This is about the security gates for the AVE trains in Spain. It is not from RENFE

https://en.astelus.com/equipaje-en-e...bir-en-el-ave/

Thanks for the feedback, I was looking up tickets on the www.thetrainline.com, so prices quoted in dollars. I like the suggestion of buying ticket at the train station, but I worry what if trains are full that afternoon? Is it a possibility?

Another thought was - can I buy a flexible ticket for last train on that day and change it to the next train as we arrive at Atocha?

Again, thanks for the info!

Perhaps, RENFE webpage would be more useful..it is in English !

Again I refer you to the RENFE website.It sets every out for you.
https://www.renfe.com/es/en/viajar/p...ia/all-tickets

Thanks, it looks like I will be getting those Elige tickets to be able to minimize risk