His 14-year-old son was with friends travelling from Billingshurst to Horsham to celebrate his birthday at Nando’s when the attack happened on Monday (May 23) at around 4pm.
The gang of four boys and two girls punched one of the boys in the back of the head and face, snatched his mobile phone and said they were armed with a machete.
The dad, who is not being named to protect his son, said the gang were wearing hoodies and masks.
“These boys have probably done it before and they will probably do it again,” he said. “I just want people to be aware.”
He said one of the girls later handed the victim back his phone.
"My son said he couldn’t phone the police at the time because the gang were all standing around him and they were stuck in their seats.”
Rail officials eventually intervened and ordered the gang off the train while his son and his friends went back to Billingshurst, missing out on his birthday treat.
British Transport Police confirmed that they had received a report of an attempted robbery involving a group of youths on a train near Horsham.
A spokesperson said: “Enquiries into the incident are ongoing and anyone with information is asked to contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 774 of 23/05/22.”