I'm going to be very curious to see what gets built. Current law is very.... Odd when it comes to emissions compliance. As long as a 'rebuild' keeps the content under X percent, then a 'rebuilt' locomotive only needs to meet the emissions regs in place when it was built. And the definition of 'rebuild' gets very technical. For example, electrical switch gear that is original parts but new built counts as 'rebuilt' - there was no upgrading of the locomotive. The key is, there's some wriggle room for not messing with existing equipment, but the goal is to eventually outmode everything so it's upgraded. D&S's locomotives, by rights, ought to simply be new build and accept the emissions controls, but I speculate there will be some creative counting of content to make them 'rebuilds'.
The rules are by no means clear, honestly, and highly open to interpretation at some points. What's crystal clear is the very high cost for non-compliance if you're found to be operating a non-compliant locomotive.
SRK