I've been thinking about the issue at hand. Adding to the next GE C21-EMP order is not an option. These still use the FDL engine, and to my knowledge GE never qualified them to currently required Tier 4 emissions even with DEF, replacing them with the HDL/GEVO line, which you would only need a 4 or 6 cylinder. GE did design an FDL-6, but I'm not sure it ever was in production and again was not known to be Tier 4.
Both MPI (Boise, part of Wabtec/Wabco merging with GE loco) and NRE have built 42" gage locos for Australia in the last few years for their open access grain trains using the Chinese knock off of the GE761 traction motor. A buyer has ordered more from NRE as there is a dispute over the MPI engines. Anything actually new (assuming you could afford it) would likely have a CAT or Cummins engine to Tier 4 standards, which has created maintenance and reliability issues due to the emissions equipment and custom designs. If you get too extensive in a rebuild, a locomotive has to be at least upgraded in emissions (which there are some interesting options) all the way to Tier 4.
Here as an interesting option as they have been advertised for sale and would generally fit the operations at the D&S, C&TS or the Loop:
https://www.locomotives-for-sale.com/Rolling-Stock/Locomotives-Multiple-Units/General-Electric-UM6B-Bo-Bo-Narrow-Gauge-Diesel-Electric-Locomotives/
These are the GE modified design for the 24" gage South Africa Railways (that replaced the famous Beyer-Garretts). The spec sheet says there are 3 available and they have 19,000 pounds tractive effort each, so it would take a pair to replace a K-36. They do have multiple unit and dynamic brakes. They would have to be rebuilt to change the gage to 36", remove the vacuum braking system and install correct couplers. Several have already been rebuilt to 42" and resold in South Africa. The trucks and traction motors are unique to the 20 that were built, so you would need to buy spares from them. These have gear units, so anything over 30 mph and they will explode the traction motors.
It looks like a US dealer is advertising the same engines:
http://www.sterlingrail.com/classifieds/classified.php?id=12052
