The discussion of MR-90 reuse reminds me quite a bit of the discussion in the Amtrak forums of Acela-1 reuse. The consensus there seems to be that the Acelas will be scrapped. Highlighted problems included their non-standard electrical systems, lack of traps, and permanent coupling. None of those seem to be an issue here - the trains are equipped with traps, are operable in normal commuter configurations, and weren't a testbed for experimental tilting and performance technologies. I don't fundamentally see a reason that no operator would be willing to buy them and get another decade or more out of them if they were to be put up for scrap pricing.
The main limitations seem to be their small fleet size (58 cars), their maximum speed, their age, and their use of 25 kV 60 Hz power.
Speedwise, they are limited to 68 mph in service and accelerate at 1.5 mph/s; decelerate at 2.5 mph/s. They weight 226k lbs per pair with 1524 hp per pair. For comparison here is what I could find about American EMUs: Arrows IIIs are 9 ft 11.5 in wide, max 80 mph, and 1125 hp per pair with a weight of 250k lbs per pair; SL IV is max 100 mph, 240k lv per pair, 1100 hp per pair, 9 ft 11.5 wide; SL V is max 100 mph, acceleration and deceleration up to 3 mph/s, 290k lb per pair, and 10 ft 6 in wide. M8s are 290 lb per pair, 10 ft 6 in wide, and run up to 80 mph.
Who would that operator be? Here's a full list of every possible candidate, as far as I know: SEPTA (or PennDOT); CTDOT (SLE / MN New Haven); NJT; RTD (Denver); Caltrain; MARC; MBTA; Mexico City. Pardon me for going a little off topic, but I'll try to keep my tour focused on the MR-90s and Exo's resale options.
SEPTA is currently planning a replacement of their 1970-vintage Silverliner IV cars. Reuse of MR-90s on this system was suggested in the corresponding forum. However, I am suspicious of this proposal, simply because SEPTA runs 12.5 kV 25 Hz power (PRR and Reading legacy) and the MR-90s are equipped for 25 kV 60 Hz. Conversion to 12.5 kV 25 Hz power would require substantial modifications and I believe would require substantially larger transformers to be installed, which would further decrease the speed and performance of the trains. If this conversion was feasible, they would fit well in SEPTA's fleet strategy because they would allow SEPTA to space out the replacement of the SL IVs over a longer period, resulting in smaller, more regular purchases. However, I would say reuse on SEPTA is very unlikely simply because of the electrical incompatibility (and if this problem could be solved than reuse would become possible).
I can't imagine PennDOT using the MR-90s for Harrisburg service given their low top speed.
NJT is currently procuring many BBD MLV EMUs to replace their Arrow IIIs and some of their older locomotive-hauled carriages. If they were not already fully engaged in this procurement than the MR-90s would have some appeal. While a significant amount of NJT's mileage is on legacy PRR wires and has high operating speeds, the Hoboken-based electric lines run 25 kV 60 Hz and have close stop spacing and low maximum speeds. The MR-90s could easily take over these routes from the Arrow IIIs. The Arrows must be taken to a shop to convert them between the Hoboken-based and Penn-based electrification systems, and one major benefit of replacing them is that this problem will be alleviated, so isolating MR-90s to the Hoboken-based system would definitely be seen as an irritation by NJT. (If 12.5 kV 25 Hz conversion was possible, they would make a good fit for the Princeton Dinky, which will be in search of rolling stock after the Arrows are gone since the MLV EMUs will run in minimum 3-car sets.) Since NJT is currently in procurement and the MR-90s could be used as-is on their system, and since the MR-90s are BBD (which NJT currently loves), I would say that MR-90 use on NJT is possible, but somewhat unlikely.
CTDOT runs a 12.5 kV 60 Hz network on the New Haven line and runs under Amtrak 25 kV 60 Hz from New Haven to New London. MR-90s could run from New Haven to New London without modification, and a conversion to 12.5 kV would be substantially less intense than a conversion to 25 Hz, so I could imagine MR-90s running on Shore Line East, either limited to New Haven or down to Stamford. However, the MR-90s would not be able to run to Grand Central (no third rail / DC) or Penn (12.5 kV 25 Hz), so they would need to be kept on SLE service, and CTDOT has M8s on order to run on SLE. In fact, the obstacles to running under wire on SLE are issues with Amtrak certification of M8s and pricing of power (neither of which would be resolved by MR-90s). The one circumstance where the MR-90s could come into play would be if CT was so desperately out of money that they could not exercise M8 options, but in this situation I imagine they would turn to further maintenance of their diesel power before they purchased the aging MR-90s, so I would say that MR-90 use in CT is moderately unlikely.
Caltrain is currently building a 25 kV 60 Hz electrification system, and they are ordering new rolling stock to replace all of their current locomotives and coaches. They are saving some of the newer stock (some of the 25 bilevels built 2002 or 2008) to run to Gilroy, and, if I was to guess, sending the rest of the bilevels (including the 16 cars from 1996) back to LA or to some smaller operator, and sending the galleries to Metra. Caltrain's 93 galleries were built in 2000, or 1985 with a 2000 renovation, whereas Metra is still running about 350 Budd galleries built prior to 1980, with 175 from the 1990s and 300 from the mid 2000s. Putting that aside, it is pretty clear that they are investing in a new, uniform fleet and will not be buying up a small number of older, unlike cars. If they need more trains, they can return to Stadler for more. Overall, reuse on Caltrain is very unlikely.
MBTA has dismissed the idea of running electric locomotives or EMUs repeatedly, even though the Providence line is electrified, because they don't want a fleet that can't run on their other lines. The Army seems to have convinced them that a build of South Coast Rail from the Stoughton branch would require electrification, which would presumably be at 25 kV 60 Hz, and the MBTA is short on cash, so if this were to move forward I could see the MR-90s being used as an interim rolling stock. In the long run, an ALP45 like solution would help them maintain fleet compatibility during an electrification build-out and make an NSRL easier, but it is pretty clear that is not happening at least until 2022 if Baker leaves office then. However, I can't imagine the MBTA moving forward with a full-build electrified South Coast Rail when other budget needs are more pressing. And unless Exo is letting the MR-90s go for virtually free, I don't imagine MBTA building out substations. So I would say reuse on MBTA is unlikely, but the MBTA has made stranger procurement decisions.
MARC is hesitant to run electric locomotives because of Amtrak's electrical rates and because they do not want to maintain a small fleet that can only run on one of their lines, just like MBTA. (This is why they did not order ACS-64s.) I cannot imagine they would be happy with not just some of their locomotives being restricted to only one of their lines, but the passenger carriages themselves as well! Furthermore, there are no plans to electrify any new lines, and the Penn line is 12.5 kV 60 Hz. So I would see reuse on MARC as virtually impossible.
RTD does not run over 79 mph and their Silverliner V cars have less powerful motors, so they are reasonably similar to the MR-90s in performance. However, Denver already has 66 cars. How many are currently in service on the A, B, and G lines; how many more will be needed when the N opens? I assume that the current 66 car order was intended to cover all of the service on these four lines (though they are now an orphan fleet with SL V out of production), so I would suggest that reuse on RTD is very unlikely.
Mexico City runs trains at maximum 40 mph in service and uses 25 kV 60 Hz. According to Wikipedia Espanol, their trains are 9 ft 10 in wide, so it is not impossible they might purchase the MR-90s used if they were to expand cheaply. (9 ft 10 wide cars and 10 ft 6 wide are run together in the US.) However, their current trains are newer than the MR-90s (operation began in 2008) and I can't find anything about expansion in progress, so I would suggest that reuse in Mexico City is very unlikely.
I'm not aware of any potential commuter operators who would build out wires (very expensive) only because they could get a deal on the MR-90s, so I'm dismissing all current diesel and DC operations.
Put together, I suggest that it is well within the realm of possibility that at least one of these improbable events occurs and the MR-90s see some continued use after REM comes online, though it is firmly possible as well that no operator would be interested in purchasing these older, mechanically incompatible units and they could end up being scrapped.
By contrast, it seems possible, perhaps even likely, that NJT will purchase the ALP45DP locomotives and MLV coaches from AMT/Exo. If not, I could perhaps see them going to SEPTA if the Phoenixville service is really going to operate; nowhere else on this list is interested in a small fleet of dual modes in the near term.