Hi Jeff,
"derailment" happens when there is a train scheduled to enter from a point, say "A" and that point is not present in the layout.
Or when scheduled to exit from point "B" and that point is not present in the layout.
For entry/exit points, the point must be linked to a track element; it's not sufficient to have the label present.
Or when a train is scheduled to depart from a station, but there is no rolling stock at the station at the ETD.
This usually happens because of a missed assignment of the rolling stock from a previous train;
it can be prevented by adding "Wait:" in the departing train's schedule.
Another possibility is that two disjoint tracks (one supposed to be the continuation of the other) are
not linked, so when the train tries to move from one track element to the next, it "derails" :)
These are the typical causes.
Regards,
Giampiero Caprino