On a superelevated curve, is the track tilted about some line that runs along the inside of the curve, or is the track tilted about the center line. In other words, when going into the curve, does only the outside rail go up, or can the inside rail go down as well? - - -pat moore Pat - Track is designed with one rail designated as the line rail and the other as the grade rail. Except in unusual conditions, North American practice calls for the grade rail to be kept at the designated grade, and the line rail to follow the designated line plus half of the gauge. Typically, in double track territory, the outer rails are the line rails, except in curves. In most curves, again except in unusual conditions, the inner rail is the grade rail, and the outer rail is raised above the grade rail to obtain superelevation. Sometimes overhead clearances call for some "shoe-horning." Curves in medium and high speed main tracks normally have "easements" at each end where the curvature gradually increases from tangent to the full degree of curvature. The length of the easement is determined largely by the amount of superelevation in the curve and the desired rate of superelevation run-off, or the rate of change in crosslevel over a designated distance, usually 31 feet. As a safe rule of thumb, I generally tried to keep the rate down to 1/2 inch per 39 foot rail, but in some lower speed curves on the D&H we had to go to 3/4 inch per rail. That was well within the present limits of the Federal Track Safety Standards. The easement may be designed to follow one of several types of spirals, or curves of varying radius. The classic AREMA "Ten-chord spiral" is based on a mathematical representation of the clothoid. Spiral easements maintained by automatic tamper-liners, or by a process known as string lining, give a curve with a uniformly increasing middle ordinate from a chord of designated length, usually 62 feet, to the gauge side of the high (line) rail. That curve closely resembles a segment of a cubic parabola. The easement should provide equilibrium at any point on it for equipment entering or leaving the curve at equilibrium speed. For equipment operating at other speeds, the superelevation underbalance or overbalance would increase from zero at the TS (Tangent-Spiral Point) to maximum at the SC (Spiral-Curve Point), where full curvature and superelevation are reached. Some curves with short easements have some of the superelevation run off onto the tangent. This gives a problem where a train exceeding equilibrium speed enters the curve from the tangent by riding against the low rail, then somewhere in the easement the flanges cross over and hit the high rail. This tends to throw the track out of line, especially if the speed of those trains is nearly the same every time so that they come across the rails at the same point, as in passenger service. The photo of the curved crossovers at Port Morris shows that each of the three tracks is at a different elevation, increasing from left to right. It looks like the right-hand track was raised so that the rails of both main tracks would be in nearly the same sloped plane to enable construction of the partially-seen curved crossover in the foreground. The curved crossover in the background shows a superelevation transition in the crossover track between the frogs to accommodate the lesser superelevation in the side track. That would have been typical of DL&W practice. By the way, a former Engineer Maintenance of Way on the DL&W, John Hiltz, later became President and General Manager of the D&H. He was an expert in the field of curve lining, and he trained some very expert curve liners on both railroads. He was known to have taken his string to some curves on the D&H even after he became President, and he also provides some more list content :-) That's enough on the subject for one day. Gordon Davids The Erie Lackawanna Mailing List http://EL-List.railfan.net/ To Unsubscribe: http://Lists.Railfan.net/erielackunsub.html ------------------------------
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