My students always hate it when I say it depends, but the answer is, it depends. First off, on the jurisdiction. As I mentioned, Georgia is one of the stingiest states on actions like negligent infliction of emotional distress. Others make it easier to sue. I did think about worker's comp as a possible avenue of compensation for crew in this kind of case, but, checking on Georgia's workers comp rules, they again require that the on-the-job caused PTSD was caused by a physical injury that triggered the PTSD. So that wouldn't work, either; and that wouldn't in any event result in recovery from the deceased person.
In states that are more generous than Georgia in allowing negligent infliction of emotional distress claims, the plaintiff crew would still have very much an uphill battle in suing the person killed. For example, family members who see a loved one killed have the easiest time prevailing, as you might guess. But even then, there's a recent case in my comparatively generous jurisdiction where a parent raced to the scene of an auto accident, having been told that her son had been killed, and saw his dead body. She sued for negligent infliction of emotional distress, and the appeals court threw the case out, saying since she hadn't seen her son killed, just seeing the dead body was legally not enough. It's a tough, tough cause of action to prevail upon.
Intentional infliction of emotional distress is a bit easier--there's a case where a guy committed suicide in the kitchen of an ex-friend, and the court said that the jury could properly find that his intent in doing it was specifically to cause the resulting emotional distress. But of course, in a train incident, there's unlikely to be evidence that the intent of the person hit was to cause distress to the crew.
No one doubts that the impact on crew of being involved in a fatal incident is horrible, and I can only imagine how awful that might be. ( I know a police officer who shot and killed someone in an absolutely justifiable case, but even many years later, he suffers terribly.) But, as I often have to tell my students, the law does not provide remedies for every harm, even harms that are serious. Cases of pure emotional distress--whether intentionally or negligently inflicted--have the least odds of prevailing, as compared to torts involved physical bodily harm or loss of property.
PS I'm not and have never been a tort lawyer, though I was a litigator in my pre-teaching life, but having taught torts and advanced torts for so long, and having former students on both sides of the aisle in the tort world, I'm probably more opinionated about torts than most!