What must a crew do when a locomotive is not going to be used for an extended period in high temperatures?

When a locomotive is not going to be used for an extended period in high temperatures, the most appropriate action is to shut it down completely. This practice helps prevent potential thermal-related problems that can arise from leaving the locomotive running for too long, such as overheating and damage to the engine and other systems.

In high temperatures, engines can run hotter than usual, and leaving them operational increases the risk of failures or maintenance issues. By shutting the locomotive down, crews can ensure that it is not exposed to unnecessary heat for elongating periods, which could lead to degradation of components. Additionally, a complete shutdown can enhance safety by reducing the risk of fire or other hazardous conditions associated with running engines in extreme heat.

The other options either maintain unnecessary strain on the locomotive systems or don’t adequately address the risks associated with high temperatures.