Complex sentence: Visitors used to be able to ride a tour train, which has been closed since autumn 2001; the closure is due to post- Sept.11 "security theater", the deteriorating conditions of the tracks, and a small derailment of the train.
Split into simpler sentences: Visitors used to be able to ride a tour train, which has been closed since autumn 2001. The closure is due to post-September 11 "security theater", the deteriorating conditions of the tracks, and a small derailment of the train.