Ticket collector Scott Lewis was accused of telling the male passenger, who was in his early 20s, to act "like a man" and insisting he look him in the eye as he sat sobbing in his seat.
The argument flared when the passenger, whose identity is unknown, tried to get a free ticket as part of a Ryanair promotion while travelling to Prestwick Airport in March.
However, the Rail, Maritime and Transport (RMT) union, which yesterday announced a strike over the incident by ScotRail staff on the last Saturday before Christmas and Christmas Eve, insisted last night the passenger should have had his travel pass validated at a ticket office before boarding the train.
It added Mr Lewis was correct to question him. The union has rejected claims the ticket collector was intimidating, insisting he had simply tried to help the passenger. Rail industry sources have confirmed details of the complaint, which led to Mr Lewis being sacked for gross misconduct following a disciplinary procedure.
Although there is no audio recording of the argument, which lasted more than 15 minutes after Mr Lewis called a booking office for advice, the description is said to fit with CCTV evidence.
The passenger is said to have described his treatment as "despicable and degrading", claiming he was not allowed to state his case in the face of threatening behaviour from Mr Lewis.
Ian Macintyre, Mr Lewis's union representative and the RMT's regional organiser for Scotland, conceded the passenger had been crying during the argument but said he had been "overplaying" his reaction.
He said: "I don't know what kind of individual he is. I know that looking at the CCTV video, he's about 6ft or 6ft1 – he's not exactly a wee thing.
"The passenger was in tears, but I don't know if he can just turn it on and turn it off again. I do know he shook hands with Scott before he got off the train."
Mr Macintyre denied accusations the ticket inspector had told the passenger to act "like a man" or demanded he make eye contact with him.
"A number of staff have told me the training they are given is to sit down and look at the passenger at eye-level to make them feel more relaxed. That's exactly what Scott has done," he added.
The RMT balloted for strike action last month after accusing ScotRail of having "not a scrap of evidence" for its claims Mr Lewis was intimidating and aggressive towards the passenger.
The train company has rejected this, pointing out CCTV footage showing the argument was shown to Mr Lewis and the RMT on three occasions.
l The people of Shetland and Orkney are facing ferry strike disruption, due to start today, over the festive season because the RMT union is flexing its muscles ahead of the tendering of the Western Isles service, Shetland MSP and ex-LibDem leader Tavish Scott has claimed.