The phrase third rail is a metaphor in politics to denote an idea or topic that is so "charged" and "untouchable" that any politician or public official who dares to broach the subject invariably suffers politically.
The third rail in a train system is the exposed electrical conductor that carries high voltage power. Stepping on the high-voltage third rail results in electrocution, in many cases leading to death. The use of the term in politics serves to emphasize the "shock" that results from raising the controversial idea, and the "political death" that the unaware or provocative politician would encounter as a result.
Disagreement may occur over whether a specific issue is a "third rail" issue. Nonetheless, the following examples are considered by some to be "third rails" in the American political system:
- Eliminating or radically reducing the scope of a major social program, like Social Security or Medicare
- Significant reform of the prison system, or the criminal justice system
- Eliminating or scaling back the War On Drugs
- Calling for a mandatory National Identity Card
- Suggesting that references to God be removed from public artifacts (coinage, etc.)
- Re-establishing ties with Cuba
- Overpopulation
Different countries have different third rail issues. In Canada, for example, the government-run healthcare system has historically been considered an "untouchable" issue for most politicians, especially those on the right.