Final answer:
Operating a train with the master controller in the switching position for prolonged periods can cause engine overheating. This is because the switching position leads to excessive power use and heat generation that can't be sufficiently dissipated, potentially resulting in engine damage.
Step-by-step explanation:
A train must not be operated for long periods with its master controller in the switching position because this could cause the engine to overheat. The driver's role is crucial in keeping the engine running efficiently. When the master controller is held in the switching position for too long, it may lead to excessive use of power, which can generate a lot of heat. This excessive heat is not easily dissipated, especially in the compact environment of a locomotive's engine, potentially leading to overheating.
In comparison to other options provided such as brakes failing, wheels locking, or lights flickering, engine overheating is the most direct result of prolonged use of the switching position. Hence, proper operation and adherence to safety protocols by the train driver is essential to prevent such occurrences and ensure a safe and functional engine over long periods of operation.