Well I live on the other side of the world from the Powder River Basin, but I am fortunate enough to have several DVD's showing and uninterupted 24hrs of operations from the PRB and surrounds. I can say from my observations that its about a 80 20 split, that is 80% for bn exec scheme and 20% for the heritage scheme 70macs, Recently a lot of 70macs have been layed up on the PRB trains due to the down turn in coal traffic, also more new 70Aces are going onto the PRB coal trains.
Hope that helps
Cheers
Lots