This has always been one of those great mysteries of life for me, since I come from an area where 40 degrees is considered cold. Why didn't the tanks and standpipes freeze up. I guess the answer is where there was enough water supply, like Carlos described, you just kept the water moving. But presumably some places like La Boca didn't have enough water to keep enough of a flow, so a bit of heat was needed. Here's the tank at Durango....like Osier maybe it was the leaks that kept the water moving.
JBWX