third rail

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But personnel benefits are a "third rail" issue in Congress - untouchable, she said.
但是軍人的薪水是國會的一件“撓頭之事”——幾乎不能觸及。
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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，軌到軌放大器的輸出狀態表現出影響開環增益從而影響閉環增益精度的負載增益傚應。
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.
根據美國環境署資料，高鉄和汽車或者飛機相比每公裡使用能源最少，而一個全國範圍的系統將能每年減少使用1.25億桶的石油。

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.