For decades, EMD has used either SD## or GP## as the prefix for its models. It seems that EMD is reluctant to go to a three-digit number (e.g. SD100), so all their new units in the last 15 years (basically since the SD90) have been "SD70ACe" followed by a suffix. That continues with the latest unit, designated an SD70ACe-T4. As you probably guess, the "T4" stands for "Tier 4", and indicates the unit is EPA Tier-4 compliant.
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