A mother and her young son were the target of abusive and racist language on a train between Leeds and Bradford.
The incident took place on Saturday, 25 April shortly after 2pm on a Northern Rail service between Leeds and Bradford Forster Square.
British Transport Police are now appealing for information about the incident.
PC Craig Virco, leading the investigation, explained: "A woman boarded the 2.10pm service at Leeds, sitting in a centre carriage with her four-year-old son. Soon after, a group of men entered the same carriage and immediately began using foul language, singing, and chanting football songs.
"The group then began repeatedly using racist language, some of which was directed at the young boy. They ignored the mother's requests to stop. When she stood up to leave the train at Shipley, she again challenged the group's behaviour, only to become the target for more abuse."
Officers have successfully isolated a number of CCTV images of men they wish to speak to as they are believed to have information about what happened.
PC Virco added: "As is understandable, the mother left the train hugely upset, shocked and disgusted at the vile abuse directed and her very young son, and promptly contacted police.
"Now we are calling on member of the public to help. Were you on the train in question? If so, we want to hear from you."
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference PSUB/B9 of 06/05/2015. Information can also be passed anonymously to the independent charity Crimestoppers on 0800 555 111.