Early Standard Gauge Track Spirit
Level for Measuring Rail Superelevation on Curves.
In order to help compensate for the undesired effects of centrifugal force on trains as they follow curves in the track at speed, the outside rail on such curves is "superelevated" or raised above the level of the inside rail on access and main running tracks on which, in general, the speed limit is 20 miles per hour or more. While the lower or inside rail holds the established grade of the track, ballast is added under the ties supporting the upper or outer rail thereby "tilting" or "banking" the track toward the center of the arc of the curve.
The amount of superelevation employed is derived by the equation E = CDV2 where:
= the superelevation of the outer rail in inches
C = 0.0005 (for curves less than 3 degrees) or
C = 0.0004 (for curves of 3 degrees and above)
D = the actual curvature of the track in degrees
V = the maximum allowable speed in miles per hour
(Click on the thumbnails to see the images full size.)
One method to correctly superelevate the outer rail on a curved track using the track level illustrated above is as follows:
With the track laid on a good surface, all the low joints on the inner rail of the curve are caught up. To first level the track, the fixed edge plate (a) of the track level (above) is rested on the top of the outer rail, and the adjustable edge plate (d), with the graduated "slide" (c) collapsed so that it is flush with the level, is rested on the top of the inner rail. When perfectly level the bubble (b) of the spirit level will rest in the middle of the tube.
To then establish the correct superelevation of the outer rail the 8" graduated slide (c) of the track level is adjusted and locked to the value predetermined by using the formula explained above. The outer rail is then raised with a track jack and ballast thoroughly tamped under the ties until the bubble of the spirit level again rests in the middle of the tube. (If necessary the elevation of the outer rail may then be increased by up to 1/2 inch in excess of that required in order to provide for settlement.)
In dressing the track after it has been superelevated the "crown" of the ballast should not be more than one-third of the width of the gauge from the outer rail in order to secure drainage. The raising of the outer rail reduces the outer slope and increases the inner slope of the ballast. If the curve is sharp, the ballast on the outer half of the track is practically level and holds water, instead of shedding it. By crowning the ballast as directed, thorough drainage is insured.
In the example illustrated in the composite image at the top of this page the outer rail has been superelevated 5 1/4" above the grade of the inner rail. According to the formula noted above, this would be the appropriate superelevation for a track with a curvature of 6 degrees 30 minutes and a speed limit of 45 MPH. The curvature in degrees may be determined by stretching tight a 62-foot string which is being held against the gauge side (inside) of the outer rail and measuring the distance in inches between the midpoint of the stringline and a point perpendicular thereto on the face of the outer rail, 5/8 of an inch below the top. Each inch of this measurement represents one degree of curvature. —BCC
The measurement (2 1/2") of the distance between the gauge face of the outer rail and the midpoint of the 62-foot string in this diagram represents a curvature of 2 degrees 30 minutes. (Diagram not to scale.)
The technique described above was adapted from A Textbook on Civil Engineering — International Correspondence Schools (Colliery Engineer Company, 1897).
Courtesy of the Bruce C. Cooper Collection.