Complex sentence: This was not the first accident in which an overheated journal box caused an axle to break and derail a train; in fact, the first-ever train wreck involving passenger fatalities, the Hightstown rail accident of 1833, had an identical cause.
Split into simpler sentences: This was not the first railroad accident in which an overheated journal box caused an axle to break and derail a train. The first-ever train wreck involving passenger fatalities, the Hightstown rail accident of 1833, had an identical cause.