What I've observed in my travels in Germany is that if you get off at one of the first stations upon crossing into Germany, there will most likely be a couple of Bundesgrundschutz officers on the platform or station area who go up to people and ask for their IDs. Especially if the train was coming from the Netherlands. And Vienna is known as Europe's transit hub for drug trafficking. The products come in from the East into Vienna, and get dispersed to various parts of Europe from there. These checks are nothing like the immigration and custom checks done at ports of entry into the Schengen area. So I do not see how this violates the treaty. I don't recall that Schengen prevents law enforcement of member countries from asking someone for their ID. At any point.