I was wondering, if a train is approaching a grade crossing and another train is occupying that crossing, does the approaching train still have to signal for the crossing?

I ask because I was out fanning today and caught a very long UP stack train rolling fairly slowly over a grade crossing when an Amtrak appeared. The Amtrak made a station stop about 200 feet short of the crossing as the UP power passed it. Its station work completed, the Amtrak blew its horn twice and left the station, as usual. Much to my surprise, the Amtrak then when straight through the crossing with no horn AT ALL.

Is that allowed? I'm local to the area, and I can confirm that the crossing is not a quiet zone. Every time in the past when I've seen a train approach an already occupied crossing, it signaled like normal. Can anyone shed some light on this?