In the days of hot metal typsesetting, a device containing an alternate typeface or type style (such as italic), which could be shifted to. The upper rail often contained the alternate typeface while the lower rail contained the original one. Early computerized typesetting devices retained the concept of the upper and lower rail, although it became increasingly a metaphor for programming and formatting commands rather than an actual device. A line or part of a line set in a different typeface or style from the surrounding text was thus known as a rail line.
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