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train slide "downhill" toward the inner rail. If this condi- 

 tion is properly provided for there is no more pressure against 

 the rails on the curve than on the tanp^ent. But now appears 

 the difficulty that in passin^;; from the straight track to the 

 circular track the superelevation must theoretically increase 

 instantly from zero to its full value. For, considering the 

 state of things at the point of curve, it is evident that at one 

 instant the train is moving in a straight line, requiring no ele- 

 vation of one rail over the other, and the next instant it is 

 moving in the arc of a circle, recjuiring the full amount of 

 superelevation. It is therefore seen that one of the great 

 problems of alignment is to ascertain the best method of 

 passing from tangent to curve and from curve to tangent. It 

 is concerning the eas)' solution of this so often solved problem 

 that the following is written. 



Before taking this up, however, it will be well to ascertain 

 as a preliminary step the amount of this superelevation under 

 various circumstances. In the figure 



let AB represent the slope of the track. At each point of 

 contact of the wheels with the rails a certain weight acts ver- 



