|Massachusetts Bay Transit Authority commuter train showing diesel-electric locomotive hauling passenger coaches. The locomotive provides all the power for lighting, heating and air conditioning on the train. The photo is by Michael Taylor.|
A common reason for keeping a diesel locomotive idling is cold weather. If the air temperature falls below 40 deg F, the engine will begin to freeze. Diesel engines don't have anti-freeze so they have to be kept running to keep them warm. Some locomotives are now being fitted with small diesel engines specially equipped with heating systems to keep them warm.
If the locomotive is providing "hotel power" for passenger cars, and the train is required to stand in a terminus or yard between trips, it will be necessary to keep the train warm to prevent it freezing (or keep it cool in the summer), so you have to keep the locomotive running to provide the power. A way to overcome this is to provide a "shore supply". A heavy duty cable has to be connected to the train to supply enough power to keep the heating/air con/lighting going. However, not many yards and terminals have these and they are expensive to install and run.
As for fuel and pollutant savings in a modern locomotive, the Environmental Protection website describes the following example. A reduction in commuter locomotive idling by even one hour per day per locomotive, together with modern ultra-low sulphur fuel fuel and a modern low-emission engine, could result in yearly carbon dioxide emission reductions of an estimated 800 tons, nitrogen oxides reductions of nearly 170 tons, carbon monoxide reductions of about 80 tons, particulate reductions of 23 tons, and sulphur dioxide reductions of 1-2 tons.
There's more information from MJ Bradley here.