The Beast!
The MP acquired control of the Chicago & Eastern Illinois in 1963, yet continued to assign C&EI reporting marks to some locomotives until 1974.
Toward the end of its tenure as a C&EI unit, GP 35 666 leads C&EI 664 and UP U28C 2883 at Fort Worth, Texas on Sept. 1, 1974. The 666 was later re-numbered MP 2563.
You may remember the story about Santa Fe Dash 9-44CW 666. The Santa Fe re-numbered it to ATSF 599 after growing tired of crew complaints about operating a locomotive that bore the “mark of the beast” 666.