A source tells me:

The operator was using a beltpack to switch cars. This was a fairly new operator. The number of cars that can be safely used by beltpack is about 25-30 cars. The operator was switching around double that number at the time of the incedent.
The switching yard is located uphill from the interchange (where the crash took place), and there is a grade- although the grade is not known. When the proper number of cars are being switched, the locomotives are safe on level ground. As the operator was pulling out an unusually long string of cars, the locomotives went onto the downhill slope, and a runaway resulted.
This accident could have been prevented if the operator didn't go over the limit of cars to be switched, or if the operator was in the cab and was able to see that the headend had started the downgrade.

almo