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, and technically advanced rail systems in the world. engineering professor george bibel says while traveling by train is largely safe passengers should be more concerned about the state of the tracks than speeding engineers. while most derailments are caused by equipment error, rail problems are a common one. the rails can fracture from metal fatigue or they can move around and shift or anything else that moves. common ones are wheels bearings axles. reporter: coincidentally, 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. in 1943 a train traveling from washington to new york went off the tracks killing 79 people.