GE could have more aggressively marketed the fact that they meet Tier 4 without DEF. In principle, they could re-engine the Genesis units as part of a capital rebuild and meet Tier 4 while still being able to refuel at railroad enroute racks. OTOH, if Amtrak refuels from trucks anyway, DEF is a non-issue as George notes, and the DEF-based engines are probably a little more efficient because they can be tuned for power and economy while the catalyst and filter take care of the dirty exhaust (same way cars and trucks are done). The Siemens units have been in use for a couple of years now in California, the PacNW (Cascades), and recently delivered to Illinois and Brightline among others - so far no huge DEF-related problems have cropped up, at least as (not) reported here.