Brazen thieves grabbed bags, at least one holding a Christmas present, from a packed cross border Enterprise as the train was stopped at Newry station.
The three young men, after appearing to have followed passengers on to the train at the stop, stood at the head of the carriage, then grabbed several bags from a rack as the whistle blew.
The three, described as being around 20, were able to escape before the doors closed and the train moved off.
Police said three men were arrested in Newry city centre on Thursday following another theft. It is understood police are investigating a link.
The 11.20am Enterprise out of Dublin was packed with the normal busy traffic and people returning north for Christmas. It pulled in to Newry station shortly after 12.30pm.
At Newry, large numbers of people were stepping off the train, while many others got on. Eyewitnesses said they were certain the three stepped on at the same time, though there is a possibility they were already on board.
One eyewitness described how the three positioned themselves at the top of the carriage. Two had their backs to the passengers in a way that looked suspicious.
When the whistle blew, each grabbed a couple of bags and jumped off before the doors closed. The train then moved off but passengers watched as they ran off down the platform.
The three would have had to sprint through the indoor part of the station as the facility is fenced. The station is located a couple of miles from the city centre. Passengers were told they took a bus into the city.
One of the victims was a German visitor, who lost a flatpack bag. The visitor was described as very upset. Another woman was also upset as she lost a Christmas present.
But a Translink employee was praised for being kind and helpful, re-assuring those who lost their baggage. He explained the three would have been caught on CCTV and that the PSNI had been informed.
The train continued on to Lanyon Place Station in Belfast via Portadown. Police did not come on to the carriage at Portadown.
In a statement in response to questions about the train incident, the PSNI said three men were arrested following a report of a theft in the Canal Quay area in the centre of the city on Thursday.
“After carrying out patrols in the area, officers located the suspects in the Hill Street area,” the PSNI said.
The three were arrested on suspicion of theft, going equipped for theft, and possession of a Class C controlled drug. They remained in custody late on Thursday evening.
Police are still investigating and appealing to anyone who may have information to get in touch. The number is 101, reference number 885 of 21/12/23.
Translink was not immediately available for comment.