A string of alleged threats against railway staff, including one which sparked huge disruption for passengers, are being investigated by police.
On Thursday, a passenger at Manchester Victoria is said to have threatened to attack a Northern employee.
Her colleagues walked out after the incident, resulting in several trains being delayed or cancelled.
Other threats have been made by social media, including one to bomb Northern's offices, after delays in recent weeks.
John Tilley, from the RMT union, said the threatened staff member's colleagues "removed themselves from a position of danger" following Thursday evening's incident.
He said there had been "an unacceptable level of threats and abuse, due to the chaos at the station".
Staff at Manchester Victoria have removed themselves from a position of danger, as per the worksafe procedure. This is due to an unnacptable level of threats and abuse, due to the chaos at the station all week. Not a boss to be seen. #northernfail— John Tilley (@JOHNJTILLEY) December 19, 2019
Passengers posted photographs of the severe crowding that built up as a result of the disruption on Thursday.
British Transport Police (BTP) were called but no arrests were made and the member of staff involved has yet to make a formal report.
But Northern spokesman Chris Jackson said: "The aggression directed towards Northern staff last night was totally unacceptable.
"As a result of the incident our staff made the decision to withdraw from the platforms to a place of safety.
"We fully support this decision and are now providing further support to those involved who were shaken and upset by the threats made."
Recently we've received several extreme violent threats to our staff via twitter. In jest or stupidity this is completely unacceptable and we've reported to @BTP. Please remember our staff are real people who are trying their best to help you. pic.twitter.com/FSo3kJ5y0g— Northern (@northernassist) December 19, 2019
A spokesman for Northern said the company had also received a number of other violent threats online in recent weeks.
"We had an incident yesterday where one person threatened to bomb our headquarters and we've had another where someone threatened to push a member of staff under the train," he said.
"These have both been reported to the police."
A BTP spokeswoman said that while officers had been called on Thursday, "no-one came forward to make a complaint at the time".
"Officers remained at Manchester Victoria throughout the evening for reassurance. Inquiries are ongoing."