Let me cite an example; perhaps one or two other members can provide additional sidelights.
The Pennsylvania Railroad's Wilkes-Barre Branch was a mixture of double and single track extending from Sunbury to Wilkes-Barre, Penna. The double track extended from Nescopeck, where at one time, a branch drew off a lot of coal taffic destined for Philadelphia, to Buttonwood Yard in South Wilkes-Barre.
By the early 1950's traffic, while about as great in absolute terms was concentrated in fewer and longer trains, and the Nescopeck Brach to Reading and Philly about to be abandoned. The PRR removed the second track between Nescopeck and HUNLOCK Tower, about five miles south of Buttonwood., but kept about two miles north of Nescopeck as a long siding.
Traditional "double track" operating rules called for each track to be used in one direction only; movements "against the current" were allowed only when authorized by written order. The situation in this case was further complicated by the existence of a modest sized yard, known by the romantic name of Honey Pot, about two miles north of HUNLOCK.
So the Pennsy designated the second track, which was not subject to interference from yard swtchers working Honey Pot, as a running track, upon which through moves could pass. As activity at Honey Pot further dwindled, meets between the longer trains could be made using "the runner".
What a revoltin' development this is! (William Bendix)