Which type of pressure is NOT measured by the duplex air gauge?

The duplex air gauge is specifically designed to measure various pressures associated with the air systems of trains, particularly those relevant to braking and safety. Among the options provided, engine oil pressure is not included in the measurements typically monitored by the duplex air gauge. This gauge focuses on the air system pressures crucial for train operation and safety protocols, which include brake pipe pressure, straight air pipe pressure, and dynamic brake pressure.

Engine oil pressure relates to the lubrication system of the locomotive engine and is not within the operational parameters that the duplex air gauge tracks. Each of the other options pertains directly to air pressure systems that are critical for ensuring that the train's braking mechanisms function properly, thereby emphasizing the importance of the right type of measurements being taken to maintain effective train operations. In this context, the reason that engine oil pressure is not measured by the duplex air gauge underscores the division between air pressure needs and those related to mechanical aspects of the train engine.