At present, international and domestic gas detection and monitoring of targets are mainly for coal, and rarely for the subway system.

目前，国际国内瓦斯检测技术及监测目标主要是针对煤矿，基本上没有针对地铁系统的。

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Coal and natural gas are mainly used for electricity generation, whereas oil is used mostly to power transportation, yet the prices of all these energy sources are linked.

煤炭和天然气主要用于发电，而石油主要用于给运输提供能量，但所有这些能源的价格是相互关联的。

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For the past century, U.S. energy policy aimed to promote cheap and abundant electricity, and the nation achieved this mainly by burning coal.

在过去的一个世纪里面，美国的能源政策主要是致力于提供廉价而富余的电力，而达到这一目标的方式主要是通过煤炭的燃烧。

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Still misunderstood, capital spending for coal operators goes mainly for maintenance, not new mine capacity.

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But in the short term, at least, coal will remain a key source of energy for the U.S. mainly because the cost of running coal and natural gas power plants remains comparable.

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Rail freight currently accounts for only 6% of all freight, mainly bulk coal, aggregates, waste and steel.

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