- an extra rail used in some electric railways, instead of an overhead wire, for supplying power
Origin of third railin reference, refer to the danger of electrocution from contact with this rail figuratively, anything considered too problematic or controversial to freely alter or reevaluate without peril, as to one's political career: Social Security is the third rail of American politics
third railthird rail
- The rail that supplies the high voltage to power a train on an electric railway.
- A subject that tends to be avoided because of its offensive or controversial nature: “Social Security, the 'third rail' of American politics” ( Charles Stein )