I have to disagree with the nuances of a response to being "rocked". In Oregon we have had at least on incident where a person was killed directly by a rock thrown from an overpass penetrated a cars windshield and directly killed a person. That sets up a situation where a thrown object becomes a potentially lethal situation that a reasonable person would attempt to avoid. If the "rocking" incidents in PA are typically 4 oz pebbles then it could be considered just a nuisance, a 4 pound stone thrown into a 100mph train windshield could likely be quite another issue.Actually, in the case of an automobile, someone throwing rocks is entirely different, both legally and conceptually.
Negligence is failing to operate with the fully expected due amount of care. Failing to do so makes you civily liable for your actions. The rocks presented no material danger to anybody- kids lobbing rocks at lexan are basically pissing in the wind. People throw rocks at trains- this has been going on forever. Engineers are expected to be able to handle a train that is having rocks thrown at it, or ignore reports of same over the radio. Failing to operate the train in this circumstance, ipso facto, is negligent.
The kids are guilty of a variety of offenses, but are not responsible for the fact that Bostian was negligent in performing as a reasonable train operating expert should in the event that somebody throws rocks at another train.
Gross Negligence is acting in a manner that the reasonable person can expect would lead to a problem. Bostian was distracted by a distracted event in the moment; this is negligent, but an honest era that could happen to any reasonably conscientious person. So he was not grossly negligent, and is therefore not criminally liable.
Willfulness is acting in a manner intended to cause problems. I'd Bostian were to, say, decide that instead of driving the train, he would lean back, look at the ceiling, and whack the alerter with his foot whenever it went off, that would be an example of willful negligence. Were he to do something on that order of magnitude, he'd be guilty of murder. But he sure as hell wasnt there.
The issue of responsibility may not even rise to negligence if the loss of situational awareness was a reasonable consequence of the information overload from the "rocking", just as a driver loosing control after swerving to avoid a child suddenly running into the roadway.
That does not relieve Amtrak of its contractual liability from failing to safely transport its passengers.