The metaphor of the third rail means something that politically can’t be touched.†
Gerald Seib uses this metaphor in the (print-only) version of his Wall Street Journal column from February 15th.
… The new Obama budget encapsulates this picture perfectly. The budget envisions that spending on all the programs Congress argues about and votes to fund every year—including defense programs—actually will decline slightly over the next 10 years.
Meanwhile, the budget estimates that spending will rise 71% for Social Security, 72% for Medicare and 115% for Medicaid over the same period, with the increases getting bigger after that. These programs are on autopilot, and will keep eating up tax dollars unless changed.
† The metaphor comes from subways; while electric trains and trams often have have a wire overhead carrying the power, subways often have a third rail on the far side of the train that carries power (enough to kill you if you touch it).