A Southern Railway train driver was verbally abused by a passenger who entered his cab.
Aslef called the incident at Bognor Regis a “serious breach of safety”.
The man gained access to the cab as the train was in the station on Monday morning waiting to leave for London Victoria.
Aslef official Graham Morris said the passenger swore at the driver and demanded to know why the train was not moving.
The passenger took a photograph of the driver and was said to be “agitated”.
Mr Morris said he was told by the driver that he felt worried for his personal safety.
“He was upset and shaking, and decided not to attempt to drive,” Mr Morris told the Press Association.
“This was a serious breach of safety. The fact that someone entered the driver’s cab is completely unacceptable.”
A Southern spokesman said: “The train was delayed for a short while at Bognor due to awaiting dispatch by platform staff.
“It has been reported to us that an alleged altercation took place on or near the train which resulted in the cancellation of the service.”
The company is investigating the incident.
Southern’s passengers have suffered months of disruption because of industrial action, staff shortages and other problems.