The footage ran for 15 minutes Friday evening at the Boqueirao station in the southern city of Curitiba. The depot was packed with travelers at the time.
The police cyber crime unit has been notified and is trying to trace the hacker or hackers, the municipal transport company said.
A Curitiba city hall official told AFP the company with the contract to operate the screens has also been notified and told it must improve its security.
Social media lit up with screen grabs of the porn and jokes about the hack.
With a population of around two million, Curitiba is the capital of Parana state and one of the largest cities in southern Brazil.
