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(tài)表現(xiàn)出影響開環(huán)增益從而影響閉環(huán)增益精度的負載增益?zhèn)鍛?yīng)。
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(jù)美國環(huán)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(fàn)圍的系統(tǒng)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.