[Formerly: What's the purpose of the rails in the middle?]
Use of gantlet or gauntlet word for two overlapping sets of rails probably derives from the original meaning of gantlet:
"referring to a form of punishment in which people armed with sticks or other weapons arrange themselves in two lines and beat a person forced to run between them."
Gauntlet is an alternative spelling of gantlet, but it also has several definitions of its own, mostly related to gloves.
Gantlet was the preferred spelling in early use of the phrase run the gauntlet—meaning to suffer punishment by gantlet or to endure an onslaught or ordeal—but gauntlet prevailed by the 18th century. Today, most writers use gauntlet, though gantlet, which is especially common in American English, is not incorrect.
[quote from grammarist.com]
You can see gauntlet tracks today at NJ Transit's Roselle Park and Union stations. This line is shared with Conrail (NS & CSX) and the stations have elevated platforms. The outer set of rails provide freights with extra platform clearance.
One additional question, and I'm not referring to "dual gauge" where trains of different gauges share one outside rail: Has anyone ever heard of what I choose to call a SEMI-gauntlet, where instead of completely overlapping, the two tracks share a common center rail?
Kent Loudon