The MBTA's F40PH "Screamers" are identical to Amtrak's in having no dedicated HEP. Loud, smelly diesel-guzzlers they are. There's no aftermarket left for those except low-rent excursion carriers, shortlines that don't care about pulling freight with a cowl unit if the damn thing just works, and commuter rail lend-lease where somebody takes them on ultra short-term to run into the ground then discard. That's not CDOT. They need something rehabbable and game for another 10 years of service, not 2. More good-condition Geeps such as the MBTA's pending dispersals, for instance.
The P40's likely have a lot of life left in them with GE targeting its HSP-46 -platform drop-in upgrade kits at early-1990's makes like the Gennies and F59PH/PHI's. Those are the kinds of aftermarket locos that can get remade to modern standards and greatly extended service life. Taking late-1970's tech like an F40 (even from the slightly more modern 1980's and early-90's production runs) and trying to overhaul it means creating some unholy Frankenstein vehicle. It's just not worth it. It's a nearly 40-year-old make. Its time has passed, like the F10's of 10-15 years ago. So, increasingly, are passenger-class Geeps. The ones with recent or pending rebuilds will be kicking around a good while longer, but there are no longer enough parts left to take the most worn out of Geeps still in daily commuter service and put them through a 3rd or 4th extended life. The passenger aftermarket's going to drift to the 15-years-newer design and much more easily modifiable F59's and Gennies...especially when the glut of to-be-replaced Amtrak and state-owned Amtrak surplus starts hitting the market.
Good-condition Geep rehabs in the short-term, "kit"-upgraded Gennies in the mid/longer-term is probably how CDOT's going to most easily feed its escalating power needs when NHHS hits full-blast frequencies and other non-NHL/SLE service expansions like increased branch frequencies, New Milford extension, or Hartford-Waterbury enter the picture.