third rail

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But personnel benefits are a "third rail" issue in Congress - untouchable, she said.
但是军人的薪水是国会的一件“挠头之事”——几乎不能触及。
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Third, rail-to-rail AMPLIFIER OUTPUT stages exhibit load-dependent gain which affects AMPLIFIER open-loop gain, and hence closed-loop gain accuracy.
第三，轨到轨放大器的输出状态表现出影响开环增益从而影响闭环增益精度的负载增益效应。
According to Environment America, high-speed rail uses a third less energy per mile than auto or air travel, and a nationwide system could reduce oil use by 125 million bbl. a year.
根据美国环境署资料，高铁和汽车或者飞机相比每公里使用能源最少，而一个全国范围的系统将能每年减少使用1.25亿桶的石油。

Early reports from the deficit commission are touching the third-rail of politics (Social Security) and that may heighten the tax, spending and deficit cutting debate, leading to further gridlock in the upcoming lame-duck session.
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Union Pacific is currently responsible for transporting nearly two-third of all north-southbound rail traffic.
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One-third of voters say the rail plan would turn them against the tax initiative.
ECONOMIST: Infrastructure in California

a metal bar, running parallel with train tracks, that supplies electricity to some types of train

The third rail on a railway track is the steel bar which passes electric current to a train.
If you refer to something as the third rail of a political situation, you mean that it would be dangerous to make changes to that thing because people feel very strongly about it.