The CNG bus fire in Stockholm, March 2019. Photo by an eye witness to the accident.
03 May 2019

The cause of the Stockholm CNG bus explosion has now been determined


A 2-way communication radio with poor audibility, delays and the driver not noticing posted road signs - those were the underlying issues that lead up to the CNG bus explosion in Stockholm in March, according to an independent study made by the bus company Keolis. 

CTIF News wrote about this incident previously in March. Click to read that article and to also see a video of the fire development

A CNG bottle (Compressed Natural Gas) must have penetrated the roof of the bus and filled the bus with gas, according to the investigation. That is the official explanation for the massive explosive fire, from which the driver somewhat miraculously survived. 


Many problems leading up to the incident

There were a number of shortcomings leading up to the accident where a CNG propelled bus was completely destroyed in an explosive fire at the Klara tunnel in Stockholm in March, according to an independent investigation commissioned by the bus company Keolis, which the radio station P4 Stockholm was the first to report .

In the report, staff in various positions at Keolis, including the driver, were interviewed.


Sign was difficult to see

When the bus was to leave the depot, a hose got stuck in the rear-view mirror, which led to the bus leaving a quarter hour past the schedule. The driver then drove the wrong way and headed towards the tunnel - which has a clearance that is too low for the bus to pass through. The driver didn´t see the warning sign, which according to the investigation,  was difficult to perceive even at low speed.

Meanwhile, the driver was attempting to converse with traffic control via a difficult-to-operate communication radio with poor audibility.

CNG bottle penetrated the roof of the bus
Subsequently, the bus slammed into an obstruction intended to stop over size vehicles from being able to enter the tunnel. A CNG bottle (Compressed Natural Gas) was pushed down through the ceiling and gas flowed in to the bus. When ignited, the fire development was so rapid that the front window was pushed out. The four gas tanks that remained on the roof ignited and started contributing to the fire as well.

The bus company Keolis says in a statement that, among other things, it will review its training efforts and discuss with the municipality how the sign visibility at the tunnel can be improved.


The CNG bus in Stockholm after the fire in March 2019.