The drone hit the train's engine at Yahodyn, located about 2km from the Polish border. Ukraine's state railway company, Ukrzaliznytsia, said a jet-propelled drone was detected 15 minutes before the impact, allowing an evacuation order to be issued. The company confirmed that the train carrying Johnson and other officials had been at Yahodyn earlier but departed ahead of schedule due to the security situation.
"Quite possibly, the target of this drone could have been the diplomatic train, which had left the station earlier than expected because of the timetable change... due to constant threats of Russian strikes," Ukrzaliznytsia said in a statement.
Johnson, who had been returning from a conference in Kyiv alongside senior security advisers including UK National Security Adviser Jonathan Powell and former Swedish Prime Minister Carl Bildt, said: "I don't know what warped logic drove Putin to blow up a stationary Ukrainian locomotive on the Polish border this morning" and asked: "When are we going to give them the air defences they need?"
Former CIA Director David Petraeus was on board another train at Yahodyn station at the time of the attack. Poland confirmed the drone struck within 2km of its border. Bildt told the BBC that passengers on his train were told to prepare for evacuation before being allowed to continue their journey. Russia said it had struck "railway infrastructure" in western Ukraine.