Complex sentence: For hours before seeing them, one may hear the noise of multiple-unit diesel engines thundering up the canyon walls in their task to get the trains over the mountains; from the other direction, dynamic brakes are whining wildly.
Split into simpler sentences: For hours before ever seeing them, you can hear massive multiple unit diesels thundering up the canyon walls in their task to get their trains over the mountains. Then from the other direction, dynamic brakes are whining wildly.