In 1994, Conrail had an existing order for 30 SD60I's but was interested in the new AC technology. So they changed the order to 26 GP60M-I's and 4 SD70MAC test units to maintain the total unit count and still be able to fit the cost and spec parameters (money had already been approved) while getting some AC engines to test. They also placed an order with GE for 4 AC4400CW's at this time as well. After testing the EMD SD80 demonstrators, Conrail changed the order again to 28 SD80MAC's and canceled the GP60M and SD70MAC orders(the GE order was canceled at this time as well).
Sources are X2200 South and Bryan Jones and Ken Lanovich of the Loconotes yahoogroup.
Below is the updated information I received from Ken Lanovich 8/26 in regards to this fact:
Conrail had another order for 30 SD60Is in place at the time. When they became interested in testing AC power, they changed this order to the 4 SD70MAC test units and the 26 GP60Is. It was the only way they could maintain total unit count and total horsepower, while cheapening certain parts of the order to stay within the money authorized by the Conrail Board /and/ still get the AC test units. Their interest in GP60s wasn't by any intent or forethought on their part, but an accommodation to get the AC test units without additional monies.
When Conrail became comfortable with the SD80 concept, this order was again changed to 21 SD80MACs. The remaining 9 of the initial batch of 30 SD80s were supposedly leased rather than acquired outright like the other 21. The lease was probably a GM incentive to seal the first SD80 order.