When can a train use its horn in a quiet zone?

I know that there are situations where a train would blow its horn in a quiet zone (someone/something on the tracks or some other emergency or distress), but are there other circumstances that would allow or permit a train to use it's horn? Like hazardous waste or other dangerous cargo? Under special government contract? VIP passenger?

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AND...I just want to say, I have always been fascinated with the trains and the rail industry. From big machines to just the thought of all the goods and commerce as part of one vehicle coming from God knows where, going to Lord knows where. I know a lot of rail workers do thankless jobs and I just want to thank you for keeping the world moving.

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Thank you in advance.