Most often this is Fernando the track inspector who checks the line for any defects that could cause a derailment. He tends to pass in the morning hours. Early morning and late afternoons tend to be busy. Unfortunately a lot of trains run on the graveyard shift when the camera sleeps. We could put a light up to capture these trains, but it would not be long before the engine crews shoot them out after losing their night vision a few times.
Derailments can also explain lack of activity.
Of course derailments don't occur daily on the pass, but they do elsewhere on the system. If a pileup occurs four hundred miles away in the Arizona desert, it can have a dramatic impact on the rest of the system as trains are held along the way until the bottleneck is opened. One day it could be dead and the next could be one train after another as the railroad deals with two days worth of movements. Another unfortunate event that occurs regularly during the day is maintenance. When heavy track work is underway the railroad places a work window in effect which means no trains are allowed on the mountain for several hours.