I don't know how in the world they re-enacted that accident, for the TV show ... Absolutely Horrifying.
Possibly if that cop wasn't in such a rush, the car in front would have missed the runaway boxcars.
When a cop is following close behind you ... it drasticly affects the way you are driving. Either you are going much slower than you would normaly, for fear of getting pulled over ... or you are distracted looking in your rear view mirror at the cop, not seeing the runaway boxcars.