NREX SD40-4 4321. Photo by KJ Larson.

The NRE SD40-4 is the newest NRE experimental rebuild. This unit sports a conventional 16-645E3B prime mover generating 3,000 hp, NRE's N-Force microprocessors and NRE's DC traction chopper giving individual axle control, further bettering the locomotives adhesion. The DC traction chopper has been shown to increase adhesion to around the same as an AC traction locomotive.

NRE out-shopped SD40-4 #4321 in October 2013. It is unclear whether more SD40-4 demonstrators will be added to NRE's roster.