NOTES ON TRANSITION CURVES. 61 



NOTES ON TRANSITION CURVES. 



By Walter Shellshear, m. inst. c.e. 



[Read before the Royal Society of N. S. Wales, June 7, 1911.'] 



In a paper read before this Society in 1888, the author 

 pointed out the advantage of introducing a short length of 

 a cubic parabola as a transition curve between the straight 

 and a circular curve in laying out railway lines. It was 

 pointed out that when the length of the transition curve 

 was small in proportion to the radius of the circle the cubic 

 parabola was extremely simple to apply. Subsequently 

 several papers giving a very interesting investigation of 

 the cubic parabola were contributed to the Society by Mr. 

 O. J. Merfield, f.r.a.s. Since the date of these papers the 

 Railway Construction Branch has adopted a four chain 

 transition for curves up to forty chains radius, and the 

 extensive tables published by Mr. Merfield have been 

 reprinted and issued to the surveyors of that branch with 

 certain instructions for the work in the field. 



Many lines have been laid out with these transition 

 curves in New South Wales, and there can be no doubt of 

 the advantage of transitions in the endeavour to ensure easy 

 running. But the question is whether it is necessary to 

 make the transition of the adopted length and with the 

 complicated field work which is involved in the method 

 indicated. Let us see what is really required to meet the 

 case. 



When no transitions are used the necessary supereleva- 

 tion of the outer rail has to be graded up on the straight 

 before the tangent is reached, with the result that the 

 flanges of the wheels tend to slide down on to the low rail, 

 so that when the tangent is reached the change of direction 

 is accompanied by a blow caused by the sudden movement 

 of the flanges from the low to the high rail. (See fig. 1.) 



