Police are attempting to identify up to 200 disgruntled passengers who halted a train then jumped off and walked down the track.
The emergency cord was pulled in the Redland area of Bristol shortly after the service left Clifton Down station at 3.30pm on Saturday.
Many of those on board were students travelling to the Tokyo World music festival in Bristol's Eastville Park.
Great Western Railway (GWR) said the train was stopped before it arrived at Redland station due to the "unsafe" behaviour of passengers.
It had been waiting outside the station for "approximately five minutes" while the driver discussed how to proceed with a control team.
Students have complained that the service was overcrowded, with some passengers experiencing panic attacks and chanting "let us off".
After the cord was pulled, about 200 people jumped off and started to walk back along the track towards Clifton Down station.
Officers from British Transport Police (BTP), who are treating the incident as trespass, attended and moved crowds off the line and to safety.
A spokesman for GWR said: "These individuals put themselves and others at great risk, and inconvenienced hundreds of other people.
"The railway is a hazardous environment, which is why railway trespass is a criminal offence, carrying a maximum fine of £1,000.
"We will be assisting BTP in their inquiries."
He added: "The train had been stopped for approximately five minutes outside Redland station after the driver was concerned that the behaviour of passengers at the approaching station (Redland) was unsafe to themselves and others.
"He was in the process of talking to our control team about how best to proceed when the incident happened."
The spokesman confirmed the incident would be fully investigated.
A spokeswoman for BTP said initial reports suggested that 80 people walked on the line but officers saw up to 200 on arrival.
"It was established that, as the train came to a stop shortly after leaving Clifton railway station, the emergency cord had been pulled and people had got off the train and started to walk back towards the station," she said.
"BTP officers at the scene helped to get the group off the line and to a place of safety.
"We are treating this as a trespass incident - one which caused numerous delays to services in the area and could have put lives in danger - and inquiries are currently under way to identify those involved."
Speaking to student newspaper Epigram, passengers described the train as overcrowded, with people chanting "let us off".
One third year history student told the newspaper: "It was awful, I was trapped in the corner. I'm claustrophobic so I started panicking and I felt like I couldn't breathe.
"People around us were shouting to the guys nearest to the window to smash it open."
Julian Burnell of Network Rail said: "Railway lines are designed for trains, not people, and are inherently dangerous places. The actions of this group of passengers, whatever their reasons, endangered not only themselves, but also their fellow passengers. We will be investigating."