From a pollution point of view, the difference between Tier 0 and Tier 4 is stark:
HC* CO NOx PM
1.0 5.0 9.5 0.60 Tier 0 Line-haul duty regulation
1.0 5.0 8.0 0.22 "updated Tier 0" for newly remanufactured Tier 0 locomotives on line-haul duty
0.5 1.5 13.5 0.34 estimates of the average pollution level of line-haul duty locomotives in 1997
0.14 1.5 1.3 0.03 Tier 4 Line-haul duty regulation
2.1 8.0 14.0 0.72 Tier 0 Switch duty regulation
2.1 8.0 11.8 0.26 "updated Tier 0" for newly remanufactured Tier 0 locomotives on switch duty
1.1 2.4 19.8 0.41 estimates of the average pollution level of switch duty locomotives in 1997
0.14 2.4 1.3 0.03 Tier 4 Switch duty regulation
(Note that Tier 4 contains an option to meet a combined NOx + HC* level instead of the individual levels: 1.4 for line-haul duty, 1.3 for switch duty.)
(There is also a special exception for pre-1995 GE Genesis locos allowing higher NOx when more than 7000 feet above sea level, at temperatures above 105 F and barometric pressures below 97.5 kPa. Frankly, when the first regulations were written, they were carved out to basically have no requirements on the engines Amtrak was already having delivered at the time.)
The particulate matter limits in particular make an enormous difference: it's more than an order of magnitude.
Waiting on the platform at Chicago Union or Albany-Rensselaer or anywhere else with a lot of trains is going to get a *lot* better when the Tier 0 locomotives are replaced. It's not so much an issue at one-a-day intermediate stations, but at the busy stations with lots of idling, it makes a big difference.
The first locomotive emissions regs are from 1997; the second round are from 2008; I'm kind of expecting a third round in 2018, and I'm expecting them to tighten the screws on remanufacturing, since there's such a huge difference between the Tier 0 and Tier 4 standards. And diesel pollution in Chicago Union Station makes the news about once a year. The EPA standards in 2008 were designed on the assumption of incremental adoption of higher-tier locomotives into the fleet; if operators attempt to evade that by hanging on to really old engines, the rules will be changed to stop grandfathering the old engines. Amtrak doesn't own a single Tier 1, Tier 2, Tier 3, or Tier 4 engine except possibly for a few of the switchers.
Once the Chargers do start getting delivered, there will be considerably more breathing room for the Genesis fleet in any case, which should allow for additional maintenance to be done, assuming the shops have the capacity to do so.
Yes. I would personally expect the oddball P32-8s to be retired first.
Amtrak's first opportunity to start replacing or remanufacturing the P42s will be in late 2017 / early 2018 when the Chargers have been delivered to the other buyers. By then, the P42s -- those which haven't been lost in wrecks -- will be 17..22 years old. The P32AC-DMs are mostly older, although they have AC motors (which mean less maintenance), and the P40 rebuilds are even older, and the P32-8s are older than that *and* unsuitable for Amtrak's service profile.
The ideal outcome would be for Amtrak to buy Chargers (or whatever Tier 4 locomotive) slowly and continuously, retiring the P42s incrementally, so that Amtrak could maintain a steady replacement rate. This depends on Siemens (or whoever) continuing to get orders from other operators, though, since Amtrak can't generate enough orders by itself to keep the production line open on that basis.