These three SD40-2s, CR 6407, 6408, & 6406, were built August, 1977. It appears this is their maiden trip east from builder EMD at LaGrange, Ill., as they descend the grade at Slope on the Pittsburgh Line with helpers in front.
Seventeen months later, on Jan. 31, 1979, CR 6407 was destroyed: http://www.railpictures.net/photo/269453/
The EMD SD (special duty) series are a strong and reliable kind of locomotive which still serve America's rails today. They have proved themselves by clocking in several million miles of freight service.