Train splitting (dropping cars).

A train is moving down the line smoothly. I did not catch it when it happened, but I happened to look down at the indicator showing things like grades, approaching signals and approaching speed limits, and only half of my train was there. I then looked at the rear (key 3) and the front (looking to the rear). Sure enough half the train was gone. I found it about half about a half mile back from the final destination. At the place it happened I was proceeding down a 0.05% grade.
What gives? Is there a reason why, and how can it be fixed?
Thanks for any help.
What gives? Is there a reason why, and how can it be fixed?
Thanks for any help.