Question: when did central pacific railroad trains first pass over donner summit
[{'answer': 'June 18 , 1868', 'provenance': [{'wikipedia_id': '887062', 'title': 'Donner Pass', 'start_paragraph_id': 9, 'start_character': 204, 'end_paragraph_id': 9, 'end_character': 218, 'bleu_score': 1.0, 'section': 'Section::::Central Pacific Railroad.\n'}]}, {'provenance': [{'wikipedia_id': '887062', 'title': 'Donner Pass', 'start_paragraph_id': 9, 'start_character': 0, 'end_paragraph_id': 9, 'end_character': 1145, 'bleu_score': 0.919855379676574, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9558784370700365396}, 'section': 'Section::::Central Pacific Railroad.\n'}]}]