How about just thinking about the problem blew for just one second. Were putting you in charge of arresting children pot shooting trains in the Midwest along about a hundred thousand miles of rural train track.

You need to come up with a system for reliably detecting hits on any part of any train (or they will aim for areas hard to detect), anywhere on a journey and immediately notifying nearby law enforcers to converge on the location before the miscreants can get away from the area. Oh, and the system has to be simple (a criterion you set for yourself), and has to be able to distinguish hits from gravel kicked up by other passing trans or the train itself.

I’m sure you worked all that out before posting and you think it’s obvious, but please enlighten those of us not as bright.