If I were a passenger on the train, I most likely would have intervened, hopefully with the help of others. According to the law, having a youngster nearby amounts to child endangerment -- plain and simple. Also, altercations in close quartered, crowded areas have a tendency to spread, i.e. other people are likely to become involved -- call it herd mentality. Situations such as these are best nipped in the bud (quickly) before things get out of hand. Remember these two geniuses are disputing the answers on their last Nuclear Physics exam.