Yes, I was wondering if my Austrian residency card constitutes an ID card for the purpose of such bus travel, as a "passport substitute"

Does this answer your question? Can I enter other Schengen countries with EU residence permit but without passport?

Is it a residence permit issued under the withdrawal agreement? I am not sure it makes a lot of difference but I am wondering (note that residence permit holders typically do not need visas, Brexit or no Brexit).

What do you mean by "I don't legally need a passport for such travel"? That you are not required to have a passport if you have an ID card?

@Relaxed Yes, I thought that a person in the Schengen area could travel within the area, without needing a passport (aside from rules by transport companies), but maybe I was mistaken. Under the withdrawal agreement, British people who were living in the EU before 2021 were eligible for residency (5 or 10 years) without needing a visa or to maintain continuous employment (or at least, this was the rule in Austria).

@user13536357 A national ID card of an EU/EEA citizen is also a travel document. Everyone else requires a passport. A residence permit card is not a travel document. 3rd Country nationals, who are residents in the Schengen Area require their passport (as travel document) and their residence permit card when travelling to another member state.

@user13536357 The Freedom of Movement rules applies only to EU/EEA citizens (just as the Common Travel Area rules only applies to British and Irish citizens). Just because there are no systematic checks at the internal borders doesn't mean that the requirement to have a valid travel document doesn't apply.

@MarkJohnson I believe that there's a little known exception to what you say, though I don't know whether it applies in this case. If I remember correctly, there was an EUCJ case in which the court held that an ID card issued by an EU country other than that of citizenship must be recognized as proof of EU citizenship. Italy issues such cards to its residents, so an Italian ID stating e.g. Swedish nationality would count. There are some potentially significant differences here, of course: it's a residence permit, not an ID, and the UK is no longer in the EU (but it's through Brexit...).

@phoog Does that judgement explicidly include the cards where NON VALIDA PER L'ESPATRIO (NOT VALID FOR TRAVEL) is printed on that card? Council of the European Union - PRADO - ITA-BO-04004

The law hasn't caught up with the security level of modern residence permits, so you are, strictly speaking, required to have your passport with you at all times in many EU countries. Also, in Italy, hotels are required to take a copy of your ID when you check in and might not accept a residence permit on its own.

@grahamj42 It's not because 'The law hasn't caught up with the security level of modern residence permits', it because it was never intended to be used as a travel document. See points 3 and 4 of REGULATION (EU) 2017/1954

@MarkJohnson well I can't find the judgment, so I don't know.

@phoog nor could I find it. Do you have a date (or year)?

@MarkJohnson no. It is unfortunately a fairly vague memory of several years ago. Perhaps it was a national court, though I would think that such a question would normally be referred to the EUCJ.

Be aware that Croatia is not a Schengen country. So you would need your passport if you wanted to take a day trip there from Slovenia.

On a side note, I think that current residence permits have stated on them "valid only with an ID", which in this case means travel passport of the non-EU country.

@MarkJohnson I don't see how you can read "never intended as a travel document" into the paragraphs you have quoted.

@OlegLobachev My French permanent residence permit does not have this endorsement.

@grahamj42 That is not my problem.

@MarkJohnson "not intended for travel" is kind of a meaningless statement. Intent doesn't matter, only thing that matters is whether it would be practical or not. I'm sure the hammer manufacturer didn't intend for me to use it as a paper weight, but there's no reason why I can't do it.

Bureaucrats love hiding behind meaningless legal lingo but here on Travel.SE we can think from first principles and avoid the BS