Metrolink train 688 is just 17 miles out of Los Angeles and is already 35 minutes late. IT seems that SCAX MP36PH-3C 890 was recently placed back in service following an engine failure, and F125 912 was teemed with it as the "protect" unit, just in case. And that was a good move, as "just in case" occurred when 890 crapped out again before this train departed Los Angeles, resulting in a brief delay. (Santa Fe Springs, California - July 31, 2019) Thanks to "Major" for additional - and correct (unlike my supposition) - background information on this photo!