I doubt Amtrak would use "yield management" ticket pricing on a route like the Capitol Corridor. More likely, it's a matter of safety--overcrowding (along with the attendant luggage) could make an emergency evacuation very difficult, not to mention the possibility of injury to people just trying to move through the train. Years ago (long before they were reserved trains) San Joaquins at times like this were extremely crowded, sometimes having 40 or more standees per car. People would ask, "why don't you add more cars?" The answer was always "because they're aren't any."