The primary reason, for not shutting down older diesel locomotives, was less to do with water cooling, or potential freezing (on the Sunset Divisions?), than in, "what if" the engine has dead batteries (OMG!!!).

And, when the oil embargo came, and fuel prices skyrocketed, and SP started to "shut them down", these turned out be mostly RR "urban legends", and they usually started just fine, and the second generation diesels weren't affected by these cooling issues.

I had an all Campangnola 9 kilo Maseratti racing bike, I rode to work on the Dinky job, from my place in Highland Park, during the very first "middle eastern" "Gas Crisis" 46 years ago. Which is maybe just more proof, that we don't really learn from history....

That's not to say that there are not extreme conditions of cold, or safety related reasons like the need to keep brakes charged, and some of the early diesels that had some of these cooling related problems, that are exceptions. These were just not the "show stoppers" that folks raised on the RR myths of that generation, had conjured them into....to shutting down unnecessary idling locos, costing million in wasted fuel costs.....

And so, to save millions of dollars, in response to "price signals" sent by the competitive free market, the SP decided to shut down a lot of idling locomotives, not in immediate use, at places like Taylor, and at other terminals, and on things like local power, at the end of shifts.

And, the first thing you noticed is how much quieter is was, and the second was how your skin didn't burn from the nitric acid, formed from the exhaust of 300 diesels idling at night, in the fog along the LA River... Which meant you didn't have to cover your car at night working at Taylor, to keep the paint from peeling.

There were a few occasions when they had to get the service truck out to start the occasional engine with a bad battery, and I even sidled up to an engine at the Shops once, where we "got the cables out" and "jump started" a 2500 with a low battery.

But, all in all, nowadays, safety or extreme conditions aside, RR's today, shut down locos when not in use to reduce avoidable costs.