This paper reports a survey of staff of Oslo Sporveier who had been involved in serious incidents such as accidents resulting in severe personal injuries and death, suicide, robbery and assault. The survey was carried out to evaluate the efficacy of Mental First Aid in a retrospective study. The introduction of Mental First Aid was based on the beneficial effects from early intervention reported from follow-up studies after previous disasters, so for ethical reasons there was no control group. When help was given within the first few days without the use of any external specialists, and with strong support from colleagues during the first days back at work, there was little absenteeism. The beneficial effects of early intervention are stressed as is the use of non-professionals in forming a social network around employees who have been involved in traumatic incidents. Safety ombudsmen who have been given brief instruction are brought to the site of the incident to apply the principles of Mental First Aid. These secondary preventive measures are very effective and could be implemented by other medium-sized transport companies whose staff are exposed to the risk of suicide attempts.