DACS blamed for MCI / CSX train problems last week

According to Computerworld, the MCI outage affecting CSX rail lines last week was caused by DACS problems. http://www.computerworld.com/home/print.nsf/all/000505DCCE This isn't the first DACS problem nor is MCI the only carrier affected by DACS problems. Bell Atlantic had a multi-day DACS problems, I've experienced 18+ hour MCI DACS problems in the past. What is it about DACS systems which seem to lead to such catastrophic problems? How can you put an entire train system out of operation due to a single piece of equipment?