The last AC testbed locomotives constructed before series production of AC power began were the SD60MACs. Four units were built in 1991 and 1992. All worked on BN wearing BN numbers (9500-9503) and a special green and white paint scheme. They were only demonstrators, and the model was not intended for large-scale production. They achieved their purpose, resulting in large sales of the SD70MAC to BN and later BNSF. All SD60MACs remained owned by EMD. They lasted in service through the BNSF merger, but were returned in 1998. Since then, they have operated on the Transportation Technology Center test track, supplying braking power to simulate loads for other engines. The SD60MAC has a 3-window cab like early SD60Ms (oddly enough, it was built in the era of 2-window SD60Ms). However, it has HTCR trucks like SD70-series engines. Turnaround: BN 9500 RailPictures.Net Photo » Burlington Northern Railroad EMD SD60MAC BN 9500 LocoPhotos.com - Photo Details - Burlington Northern - BN 9503 RailPictures.Net Photo » Burlington Northern Santa Fe EMD SD70MAC At work on BNSF, between two SD70MACs. RailPictures.Net Photo » EMD EMD SD60MAC At its current job.