The train crew member criticised the British Transport Police (BTP) response to the incident, in which she was confronted by a gang of youths hitting a younger boy with a bottle and pouring alcohol over him.
She contacted police, but was later told the officers sent to the incident had been diverted to deal with a trespasser on a line in Glasgow, and did not arrive until after it was over.
The Rail, Maritime and Transport union (RMT) said staff would boycott trouble spots if action was not taken.
Scotland organiser Mick Hogg told The Scotsman: “If the situation does not get fixed, RMT and [train drivers union] Aslef will instruct our members not to run trains into the affected hot spots/routes until staff can go to work and be safe.
"Our railway is becoming a magnet for anti-social behaviour and nothing is getting done about it.
"It’s just a matter of time before there is a fatality.”
Mr Hogg said there should be more police patrols on trains and at least two ticket examiners on every service rather than the current single one.
Chief Superintendent Gill Murray, BTP’s Scotland commander, has already warned of an “emerging threat” of youth anti-social behaviour on trains and buses.
In March, transport minister Jenny Gilruth agreed to consider an RMT call for a new law to ban disruptive passengers from ScotRail like they can be from football grounds.
In Sunday’s incident in West Dunbartonshire, the ticket examiner said: “I was aware that the notorious ‘young team’ [about 15-20 young teenage boys] boarded the service at Alexandria.
"One of them, who was wearing a light green hoodie and carrying a bottle of alcohol, headed towards three young boys and challenged one of them, then proceeded to hit one of them with his bottle and poured all the alcohol over him.
“By this point, the rest of the young team were all bundled in that compact area and were going nuts.
“Two of them were physically attacking one of the boys.
"The other two innocent boys just sat in shock looking terrified as their pal got beaten up.
“I shouted to get them off the train and no-one was listening.
“When we got to Renton, as I was physically removing some of the young team off the train, they were trying to push past me to get back on.
"However, I was stood firm in the doorway and had to shout to the driver to shut the door.
"The boys were continuing to shout abuse and trying to figure out a way to get back on.
"Luckily, the doors closed and we got away.
“The young boy who suffered the attack was too scared to report anything, but what I witnessed was a horrific ordeal.
"I feel traumatised at what I have witnessed, and I feel physically sick – my blood pressure was sky high.
"Every day I come to work I feel my safety is being jeopardised.
"This is taking its toll on my mental and physical health.”
The ticket examiner also questioned BTP’s priorities.
She said: “I am not happy with their response.
"BTP turned up, took a brief statement, and advised me they had been en-route to my incident, but were told to turn around and go to another incident at Hillington involving a trespasser.
"I feel like some unknown trespasser took priority over my own safety and that of paying customers.”
She said she had had to end her shift early because of the incident and would not be returning to duty “until further notice”.
BTP said its enquiries were continuing into the incident after being alerted shortly after 5pm on Sunday to reports of an assault on board a train.
A spokesperson said: “Enquiries into this incident are ongoing and officers are appealing for witnesses or anyone with information to come forward.
"You can to this by texting 61016 or calling 0800 40 50 40 quoting reference 378 of 1 May.
“Through partnership working with ScotRail and Police Scotland, we’re working tirelessly to deter criminality and anti-social behaviour on the railway network, and we have been increasing patrols on the Balloch line using targeted operations to deter offending.
“Where offences involving youths take place, we regularly engage with schools and campus officers to identify those responsible and deal with them accordingly.
"We will take action as the railway needs to be a safe place for everyone using it.”
ScotRail said such behaviour was “completely unacceptable”.
Security and crime manager Stephen Elliot said: “The safety of our customers and employees is our number one priority.
“Anti-social behaviour, whether that’s physical violence, verbal abuse, or any other form, is completely unacceptable.
“Everyone has the right to go about their day in peace, so we’ll continue to work with the authorities to ensure that anyone responsible for anti-social behaviour is brought to justice.”