At the risk of stating the obvious, aren't the emissions gonna be the Achilles heel? I can't cite the emission factors for Tier 4 diesels off the top of my head, but certainly that's been a big program to reduce diesel emissions. I don't know what the Amtrak requirements are...
One of the big reasons that diesels replaced steam 'way back when' was the availability issue - again I don't recall the exact numbers without digging around, but a lot of big road steam engines spent as much or more time in the shop than on the road - maintenance was a really big deal. Presumably a more modern design might be able to improve that, but steam engines are really complicated - and were often specialized or unique designs so it was difficult to to swap out parts.
I'm a big steam engine fan, but I can't see them making any sort of comeback even with modern designs and Tier4-level (and beyond) emission controls. The cities along the NE corridor long ago required electrification - and Amtrak's operations in the NE corridor have continued that. Indeed, all of the highspeed rail around the world is electrified. We're even finally gonna get electrification on the Caltrain route into SF (its been a long slow march...).
