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I have a photo showing some variant of an E60 and the two units are definitely numbered higher than 975, as I said the first two digits are a 9 and an 8 respectively, the third number is occluded. What is in my picture if it is not some E60 variant, what else looks like an E60?
Looking through several on-line rosters, I can comfortably state that Amtrak E60 units' numbers ended at 975. Amtrak did have two locomotives in the 990 series (X995 & X996) which were foreign units on loan for testing ahead of the AEM7 design:
AMTRAK 995 BOXCAB ASEA/NOHAB /76 SWEDISH STATE RAILWAY 1166--LEASED 6/76 TO 4/77
AMTRAK 996 BOXCAB ALSTOM/FRA /74 SNCF CC21003--LEASED FROM 1/77 TO 6/77
(source: https://www.thedieselshop.us/Amtrak%20all-time%20roster2.txt)