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A vertical curve is a smooth curve in the vertical plane of a railway line, used to connect different slopes. It is necessary because direct connections between slopes with significant differences in gradient can cause stress changes in train couplings, potentially leading to decoupling or derailment.

How to set a vertical curve?

For lines with a maximum allowed speed of ≤160km/h, a vertical curve with a radius of ≥10,000m (or ≥5,000m in difficult sections) should be used when the algebraic difference in slope between adjacent sections exceeds 3‰. For lines with speeds >160km/h, a vertical curve with a radius of ≥15,000m and a length of ≥25m should be used when the slope difference is ≥1‰. Vertical curves should not overlap with other vertical curves or transition curves, and should not intrude into switches, regulators, or open deck bridges.

How to mark a vertical curve on site?

The start of a vertical curve is marked on the rail with "SQ L=length of vertical curve", the point of slope change is marked with "SB R=radius of vertical curve", and the end is marked with "SZ L=length of vertical curve". The markings are in red with white text, similar to curve markings.