An explosion on a key Polish railway line used to transport weapons to Ukraine was caused by an "unprecedented act of sabotage", according to the country's prime minister. Donald Tusk said an investigation was under way and vowed to catch those responsible for the attack, which has partially destroyed the line that connects Warsaw and Lublin. No injuries have been reported, with Mr Tusk saying that "fortunately" tragedy had been avoided. The blast is the latest in a wave of arson, sabotage and