Which of the following is NOT an indicator in the engineer's operating cab for reading train line conditions?

The engine temperature gauge is not an indicator for reading train line conditions because its primary function is to monitor the operating temperature of the locomotive's engine. This gauge provides crucial information regarding the engine's performance and helps prevent overheating, but it does not convey any information related to the air brake system or the train line.

In contrast, the brake pipe pressure gauge is specifically designed to provide real-time data on the pressure within the brake pipe, which is essential for understanding the effectiveness of the air brake system. Similarly, the brake application light indicates whether the brakes are currently being applied, which is vital for the engineer to manage braking safely. The door closed light, while not directly related to train line conditions, still serves a safety function by ensuring that the operator is aware of whether the train doors are secured before departure. These indicators collectively provide a comprehensive overview of the brake system's performance, while the engine temperature gauge does not play a role in this context.