First or second class train ticket?

If you book early - like 4-6mo out, you can get VERY good deals on 1st class. We have bought 1st class very early and then checked back a couple months later and we got 1st class cheaper than what 2nd class was going for at the time - so we always book early and go 1st class. Is it necessary, NO - is it worth it, YES.

You can go to seat61.com and read all about 1st VS 2nd class and see photos.

Now, we have not been to Portugal yet -- so I am only speaking about Germany, Austria, Italy, Switzerland.

The difference is the size of the seats.
First class has 3 seats ( 2 seats / aisle / 1 seat ).
Second class has 4 seats ( 2 seats / aisle / 2 seats ).
To us it's worth paying for the extra space.
Wrote a long trip report that has some photos of the AP train, scroll to the train photos if you want to check it out.
https://www.fodors.com/community/europe/extended-trip-to-portugal-1711933/

Purchase tickets only from the official site
https://www.cp.pt/passageiros/en/buy-tickets
Tickets are available a maximum of 60 days before departure ( sometimes they're discounted).

Aulophone - Thank you immensely for the link with all the pictures. This will be so helpful to actually preview and be able to recognize when I go in September.