ÖBB (Österreichische Bundesbahnen), Austria's state rail operator, publishes its pet policy directly on its own site, and it's precise rather than vague. Small, harmless animals travelling in a suitable, closed transport container travel free — the page doesn't give exact carrier dimensions, so a reasonably sized, fully enclosed carrier that keeps the animal contained is the safe assumption, and asking ÖBB staff directly is worthwhile if you're unsure.

A dog travelling without a carrier is a different matter: ÖBB's own terms require a muzzle and a leash 'zur Sicherheit unserer Fahrgäste' (for the safety of our passengers), and the dog needs a ticket costing 10% of the full standard adult fare for that journey, with a minimum charge of €2. The dog may sit on the floor, but only under the direct supervision of its owner — ÖBB's phrasing is explicit that this isn't a shared responsibility with staff.

The one genuine exception is an assistance dog registered in its owner's Behindertenpass (disability pass): these dogs travel free and are explicitly exempted from the muzzle requirement, since the muzzle rule exists for passenger safety around an ordinary pet rather than a working assistance animal. The page covers most ÖBB trains generally rather than naming specific exclusions for Railjet or Nightjet services, so the same core rules are a safe baseline across the network.