This was Germany, where the same is true. However it's not allowed to use that as a stand-in for a border control. European Regulation is quite explicit on internal Schengen borders:
> 23(a) the exercise of police powers by the competent authorities of the Member States under national law, insofar as the exercise of those powers does not have an effect equivalent to border checks; that shall also apply in border areas. Within the meaning of the first sentence, the exercise of police powers may not, in particular, be considered equivalent to the exercise of border checks when the police measures:
> (i) do not have border control as an objective;
> (ii) are based on general police information and experience regarding possible threats to public security and aim, in particular, to combat cross-border crime;
> (iii) are devised and executed in a manner clearly distinct from systematic checks on persons at the external borders;
> (iv) are carried out on the basis of spot-checks;
This check likely broke both 23(a)(i) and 23(a)(iii), as at the first train stop past the border everyone was asked to present ID.
[1] Regulation (EU) 2016/399 of the European Parliament and of the Council of 9 March 2016 on a Union Code on the rules governing the movement of persons across borders (Schengen Borders Code)
What you've described may not necessarily be the "equivalent to the exercise of border checks".
That depends on what would have happened had you not complied. In France if you are unable to produce an acceptable ID, you're taken to a police station where you're held until your identity is established. Once established, you'd have been allowed to go your way.
It would only constitute "the exercise of border checks" if the response to your non-compliance was to escort you back to the country where you boarded the train.
I'd be surprised if that's what generally happens. One instance I observed on a train going through Switzerland involved an English-speaking student. It took a while but after some discussion and the police making a few phone calls seemingly to their satisfaction, warning her to carry appropriate ID in future.
>It would only constitute "the exercise of border checks" if the response to your non-compliance was to escort you back to the country where you boarded the train.
Not true according to the regulation:
> ‘border control’ means the activity carried out at a border, in accordance with and for the purposes of this Regulation, in response exclusively to an intention to cross or the act of crossing that border, regardless of any other consideration, consisting of border checks and border surveillance;
Note, it covers "response to crossing a border" and includes "border surveillance".
edit:
For full context:
> ‘border checks’ means the checks carried out at border crossing points, to ensure that persons, including their means of transport and the objects in their possession, may be authorised to enter the territory of the Member States or authorised to leave it;
> ‘border surveillance’ means the surveillance of borders between border crossing points and the surveillance of border crossing points outside the fixed opening hours, in order to prevent persons from circumventing border checks;
> 23(a) the exercise of police powers by the competent authorities of the Member States under national law, insofar as the exercise of those powers does not have an effect equivalent to border checks; that shall also apply in border areas. Within the meaning of the first sentence, the exercise of police powers may not, in particular, be considered equivalent to the exercise of border checks when the police measures:
> (i) do not have border control as an objective;
> (ii) are based on general police information and experience regarding possible threats to public security and aim, in particular, to combat cross-border crime;
> (iii) are devised and executed in a manner clearly distinct from systematic checks on persons at the external borders;
> (iv) are carried out on the basis of spot-checks;
This check likely broke both 23(a)(i) and 23(a)(iii), as at the first train stop past the border everyone was asked to present ID.
[1] Regulation (EU) 2016/399 of the European Parliament and of the Council of 9 March 2016 on a Union Code on the rules governing the movement of persons across borders (Schengen Borders Code)