http://www.railpower.com/First let me make it clear ALL diesel locomotives are "Hybrids" in that the diesel engine generate electric power and the electric power is used to power the electric motors on the locomotive's wheels.
The Green Goat just takes this process one step further in that a battery pack is added so that the diesel engine does not have to be on ALL THE TIME (Which is what happens in conventional Diesel Engines). Thus you do not need to Idle the Green Goat (i.e. keep the engine on even if it is standing still waiting to be used). This lack of the need to idle i.e. that the diesel engine can be OFF and the switcher can still operate, is what is producing both the pollution reduction and fuel savings.
I do not see this technology going much further in this field, switchers are notorious for operating at low speeds but at full capacity and than for brief period of time (and idling for hours at a time). Locomotives actually used to pull a train tend to operate above their idle speed for most of their trip and this the savings just are NOT there.