For COVID requirements, the best is to check reopen.europa.eu.

Requirements change often and with little notice, so what was valid last week may not be valid anymore now, let alone next week or next month.

Thank you :) 

I've actually been checking their website but nothing really seems to appear regarding PLF when travelling by rail, only by air it seems! 

That's possible and may also vary per country. I don't check the website myself unless I'm travelling because it changes so often. To be sure, you could click through to the national websites. In any case, the Reopen EU website (or app) is more reliable than experiences or opinions on a forum :)

My own country (NL) has its own site from a ministry that is even more wider and very uptodate for all requirements. I arrived 2x in ESpana-by plane, last time early febr and at airports it very certainly was checked very much. For FR no need, nor has ever been.

If you FLY to PT the airline you take will have the info.

It is also quite another thing that even if rules exist, how much they are enforced. In general though it is not very hard to do-can easily be done using free wifi on your holy fone-I´ve also seen people doing it in 2-3 mins on arr, at ES airport before the checks.

Also note that you really still need that QR code you have been vaccinated, as its more often HTL and restrt that want to see it! I even had such a request when just buying a cup of cf in an ES-station snackbar.