Most diesel locomotives use water as coolant. If the unit is left shutdown for an extended period of time in freezing temperatures, the water will freeze. This could damage the locomotive since water expands as it freezes. Most railroads have rules that locomotives cannot be shut down in temperatures below 40 degrees unless the water is drained. Since starting diesels is time consuming, they are frequently left running even in warmer weather.
Fuel consumption drops to only a few gallons per hour in idle mode.
Water is used as collant in most locomotives. There is also the problem of leaks allowing oil and anti-freeze to mix, causing a loss of much of the lubricating ability of the oil. In addition, anti-freeze can be very expensive and a large amount would be necessary for a locomotive. The new 6000 hp units from EMD and GE are capable of using anti-freeze, as well as the caterpillar engines used in some units. The amperage limit of the traction motors.