By 2011 the FCC requires that all land mobile radios be "narrow banded." A narrow band radio transmitter will put out lower volume audio in the same receiver (scanner) than will an existing transmitter. Both new and old standard radios can communicate with few problems. What you are hearing probably is a mixture of new and old standard radios. Many AMTRAK locomotives sound to me as if their radios already have been converted to the new standard. Most detectors have not been converted yet.

BTW railroads use FM (Frequency Modulation) transmitters. Signal strength received is a function of transmitter power, distance, and terrain. The audio volume within very wide limits is NOT a function of received signal strength. If the transmitters are properly aligned and the person is using good microphone technique (not always heard), the audio volume received should be the same whether the transmitter is one mile away or 20 miles away. (However if your receiver has a ratio detector and you are standing next to a transmitter, audio volume will be higher).

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