some sort of alarm went off at Wien Hbf, and everyone is told to leave the station.

Don't go there rn, and avoid its immediate radius. Many neighboring stations are also closed.

Edit: Bomb threat https://www.meinbezirk.at/wien/c-lokales/wiener-hauptbahnhof-nach-bombendrohung-evakuiert_a7182438

Edit: Around 18:00, police intervention was announced to be over. Trains massively delayed, but running.

Apparently someone sent a threat to the police but they found nothing