traveling three times the posted speed. killing four. with more than 11 million passengers traveling along the northeast corridor between washington and boston each year it has become one of the busiest, most complex, technically advanced rail systems in the world. engineering professor says while traveling by train is largely safe, passengerss should be mor concerned about the tracks, and not engineers. most are caused by equipment, rail problems. rails can fracture from metal fatigue, or move around and shift, or anything else that moves. common ones are wheels, beari bearings, axels. the site of tuesday s crash in philadelphia is in the same area where the nation saw one of its deadliest train accidents in history.