The train was waiting at Hjällbo when the incident happened. Nobody was injured.
It is still unclear what was thrown into the carriage but police say that although there was a significant amount of smoke, there appeared to be no flames.
“Someone threw a burning object in, but the fire went out by itself, so there was just a lot of smoke produced,” said Jan Lundin, who was investigating the incident for train company Spårvägen.
Lundin told TT that the train driver had seen two people near the tracks, who had had their faces covered with hoods, shortly before the object was thrown into the train.
The affected carriage has now been taken out of service.