Track Bed and Foundation
Railway tracks are generally laid on a bed of stone track ballast or track bed, in turn is supported by prepared earthworks known as the track formation. The formation comprises the subgrade and a layer of sand or stone dust (often sandwiched in impervious plastic), known as the blanket, which restricts the upward migration of wet clay or silt. This may also be layers of waterproof fabric to prevent water penetrating to the subgrade. The track and ballast form the permanent way. The term foundation may be used to refer to the ballast and formation, i.e. all man-made structures below the tracks.
Additional measures are required where the track is laid over permafrost, such as on the Qingzang Railway in Tibet. For example, transverse pipes through the subgrade allow cold air to penetrate the formation and prevent that subgrade from melting.
The sub-grade layers are slightly sloped to one side to help drainage of water. Rubber sheets may be inserted to help drainage and also protect iron bridgework from being affected by rust.
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Famous quotes containing the words track, bed and/or foundation:
“It is remarkable how easily and insensibly we fall into a particular route, and make a beaten track for ourselves. I had not lived there a week before my feet wore a path from my door to the pond-side; and though it is five or six years since I trod it, it is still quite distinct. It is true, I fear, that others may have fallen into it, and so helped to keep it open.”
—Henry David Thoreau (1817–1862)
“Professor: War is hell, Mr. Thornhill, even if it’s just a cold one.
Roger Thornhill: If you fellows can’t lick the Vandamms of this world without asking girls like her to bed down with them, and fly away with them, and probably never come back, perhaps you ought to start learning how to lose a few cold wars.
Professor: I’m afraid we’re already doing that.”
—Ernest Lehman (b.1920)
“But I hate things all fiction ... there should always be some foundation of fact for the most airy fabric—and pure invention is but the talent of a liar.”
—GGeorge Gordon Noel Byron (1788–1824)