I think at this point Tier 4 is reaching beyond existing proven technology as far as reliability and economics, neither of which regulators seem to care.

Until technology catches up, maybe the EPA should allow T3 engines continue to be built with a financial penalty (which the government can use to subsidize "green" energy, not EPA bonuses and conferences). Once T4 technology becomes proven and reliable, then economics should justify buying T4 engines instead of T3 plus the penalty. As it is now, the reduced reliability and potential DEF requirements are going to make T4 expensive beyond simply paying for the new technology.

MPI is involved in trying to adapt the new large Cummins engines into a T4 design (both new locos and retrofits). I have heard that the DEF (urea) may require as much as 12% of the fuel tank capacity. For locomotive service, this isn't going to be just top of the tank every 92 day inspection, but probably at every fueling.

I wonder if anyone every gave railroads and barges credit for their energy efficiency in relation to their emissions? Does it make sense for a locomotive to meet the same emission requirements as a truck when the locomotive is hauling 10 x the net freight per gallon? I also wonder if the regulators have figured in the environmental impact of throwing away prime movers and having to replace them with totally new designs every 5-10 years compared to 15-50+ for existing designs?