The order was on the books, but the money had originally been budgeted for 30 SD60Is. When the AC technology came out, CR wanted to experiment, and the order for the 30 SD60s was changed to 4 SD70MACs and 26 GP60I's. The remaining 26 DC units were changed to GP60s in order to maintain the total horsepower of the original 30 unit SD60 order, and still keep the total price within what was already budgeted (GP60s were alot cheaper per unit than SD60s). Basically, the additional cost of the 4 AC units was offset by cheapening the remaining 26 units from 6 axle down to 4 axle specifications.
Before these orders went into production, EMD got Conrail interested in the SD80, and the split order for 4 and 26 was replaced by 21 SD80MACs (owned, paid for with the original budgeted money set aside for the 30 SD60Is), and then Conrail and EMD agreed on a lease for an additional 9 SD80s, making the initial SD80 order for 30 (21 owned, 9 leased).
Sometime around 2010, when the lease is up on those 9, fans are gonna be scratching their heads wondering why just a few and not all are leaving the NS and CSX rosters....