Question: when are trains required to blow their horns
[{'answer': 'at all public grade crossings', 'provenance': [{'wikipedia_id': '9330997', 'title': 'Train horn', 'start_paragraph_id': 18, 'start_character': 267, 'end_paragraph_id': 18, 'end_character': 296, 'bleu_score': 1.0, 'section': 'Section::::Operation.:United States.\n'}]}, {'provenance': [{'wikipedia_id': '9330997', 'title': 'Train horn', 'start_paragraph_id': 18, 'start_character': 0, 'end_paragraph_id': 18, 'end_character': 570, 'bleu_score': 0.979484453366057, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 7399102936868551758}, 'section': 'Section::::Operation.:United States.\n'}]}]