Question: what is the fastest steam train in the world
[{'answer': '4468 Mallard', 'provenance': [{'wikipedia_id': '716938', 'title': 'LNER Class A4 4468 Mallard', 'start_paragraph_id': 0, 'start_character': 14, 'end_paragraph_id': 0, 'end_character': 26, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '716938', 'title': 'LNER Class A4 4468 Mallard', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 296, 'bleu_score': 0.7129785398119021, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 7408978822804559837}, 'section': 'Section::::Abstract.'}]}]