The correct answer is B. 64 mm.
Superelevation is the difference in elevation between the inner and outer rails of a railway track. It is used to counteract the centrifugal force that acts on a train as it travels around a curve. The amount of superelevation required depends on the speed of the train and the radius of the curve.
Cant deficiency is the difference between the actual superelevation and the required superelevation. It can occur due to a number of factors, such as poor track alignment or maintenance. Cant deficiency can lead to derailments, so it is important to keep it to a minimum.
In this case, the superelevation required for the branch line is 10 mm and the cant deficiency is 75 mm. This means that the superelevation to be actually provided on the branch line is 10 mm – 75 mm = -65 mm. This is a negative value, which indicates that the outer rail is actually lower than the inner rail. This is not ideal, but it is the best that can be done in this situation.
The following are the explanations of each option:
- Option A: 10 mm. This is the superelevation required for the branch line. However, this is not the superelevation that will actually be provided, as there is a cant deficiency of 75 mm.
- Option B: 64 mm. This is the superelevation that will actually be provided on the branch line. This is calculated by subtracting the cant deficiency from the superelevation required.
- Option C: 85 mm. This is the superelevation that would be required if there was no cant deficiency. However, this is not the superelevation that will actually be provided, as there is a cant deficiency of 75 mm.
- Option D: 65 mm. This is the superelevation that would be required if the outer rail was 65 mm higher than the inner rail. However, this is not the superelevation that will actually be provided, as the outer rail is actually lower than the inner rail.