"His lordship, having observed … that there was evidence to show that the guard was doing his best to stop the train by applying the brake, went on to remark that" when a man had a duty to perform, if he performed it negligently, and death ensued in consequence, he was liable for the consequences; but a man was not guilty of manslaughter merely because he did not do that which a stronger or more clever or cool-headed person would have done under similar circumstances.