As far as safety goes, a 50 pound limit is clearly not stringent enough. In a derailment, getting a direct hit with a 50 pound case would likely be serious if not fatal. (Actually, a much lighter weight object can cause a serious injury--many years ago, an uncle sustained a pretty serious injury in a car crash when he was hit by a flying tissue box. Really.) Obviously, there has to be a compromise between the safety consequences of flying luggage and giving people a practical luggage allowance. If we were really concerned with safety, there would need to be latching luggage bins as in aircraft. I assume, though, that derailments are rare enough events that the additional cost and inconvenience of using overheard latching bins is considered to be not worth the marginal safety gains.