When did railroads start using diesel engine block heaters?

[Puddlejumper]

I would imagine that for a short line railroad, electric engine block heaters may have been a very early development. For a small railroad it would not have been economical to leave a locomotive idling between uses nor would it have been acceptable to drain the radiator after each use, and refill it before the next use. Unless they used antifreeze in the radiators. For a large railroad, a class one, it was common practice until perhaps 20 years ago to just leave the units running all the time, especially during winter. This prevented the engine blocks from freezing in winter, as antifreeze was not used in the radiators of locomotives. It is also difficult to start a cold diesel engine, especially one THAT big. The engine block heaters used nowadays are APUs, or auxiliary power units. They have temperature sensors and sensors that monitor the state of the batteries that detect when to start the main diesel to keep it from freezing, as well as the ability to circulate the coolant to keep it warm in winter, and charge the batteries. They start up and shut down as needed.

I hope some of this is helpful.

Dave