An elderly man allegedly put a young boy in an armlock on a train because he wouldn't take his feet off a seat.
The man, believed to be in his 70s, is understood to have confronted the child on the service from Clitheroe, Lancashire, to Manchester on Friday.
Photos of the incident appear to show him bending over and holding the youngster's arm behind his back - as other passengers look on.
The images, taken by another child travelling with the boy, have since been widely shared on social media.
According to witnesses, the disagreement happened at around lunchtime on Friday on the Clitheroe to Manchester Victoria service.
Three young boys, thought to be aged eight to 11, got on the train at Blackburn and were travelling unaccompanied.
It's understood that the children had been sitting with their feet on the seats, and were approached by the man who asked them to put them down. One student, who was sat behind the boys, said the pensioner told them to put their feet down and they refused.
"They were being quite cocky," he said.
Other reports too suggest that this was the case.
The 17-year-old added: "He threatened them with a 'clip around the ear' and then went and sat back down. The children then put their feet back up and wouldn't take them down. The guy then went over again and tried to take their feet down."
It is at this point, when the train was near Bolton, that the man is alleged to have put one of the boys into an arm lock, a photo of which taken by one of the other children.
The British Transport Police has confirmed it is investigating the alleged assault, while Northern Rail is also looking into the incident.