Final answer:
A fixed signal at the entrance of a railway block that governs the movement of trains is called a signal block or block signal. It ensures safety by indicating if a block is occupied. The Doppler effect causes frequency changes observed by a stationary person as a train approaches and passes.
Step-by-step explanation:
A fixed signal at the entrance of a block that governs trains entering and using that block is known as a signal block or block signal. This system is crucial in railway operations, as it ensures the safety of the trains by governing the movement of trains into and out of a particular section of track, referred to as a block. The signal indicates whether the block ahead is occupied or free, preventing collisions and maintaining an efficient flow of train traffic. These signals are typically operated by automatic systems that detect the presence of a train within the block through the use of track circuits or axle counters.
For the stationary person at the side of the tracks, the frequencies they observe as the train approaches and after it passes are changed due to the Doppler effect. As the train approaches, the frequency of the sound (like the train's whistle) appears higher than when the train is stationary or moving away. For the train's engineer, the frequency of the sounds they produce does not change; it remains at the source frequency since the engineer is moving with the source of the sound.