Shep, train maybe waiting for the tracks to clear further along, or for another train with priority to pass by going the other direction. Just guessing!
This afternoon, 3:00 or so, a flock of swallows was active flying in and out of a round entrance on the cliff face, to the left of the nest box. There's a species called cliff swallows that occupy most of North America. Experts, were these cliff swallows?