question: What can become unreliable if the speed exceeds the limit of 100 mph for DC system trains ?
answer: contact between the shoe and the rail
turker_answer: The contact between the shoe and the rail become unreliable if the speed of DC system trains exceeds the limit of 100 mph .
rule-based: Contact between the shoe and the rail can become unreliable if the speed exceeds the limit of 100 mph for DC system trains .