Final answer:
The question lacks the necessary details, such as the radius of the main curve and its banking angle, to calculate the speed restriction on the main line curve in a train yard.
Step-by-step explanation:
The student's question pertains to determining the speed restriction on a main line in a b.g. yard, given that a 7-degree curve branches off from a 4-degree main curve in an opposite direction, with a speed restriction of 30 km/hr on the branch line and a permissible cant deficiency of 75 mm.
Unfortunately, the information provided is not sufficient to calculate the speed restriction on the main line because further data such as the radius of the main curve and the cant (bank angle) are needed.
In railway engineering, the cant deficiency is used in conjunction with the curve radius to determine the maximum speed of a train on a curved track, but the angle of the main curve or banking details are essential for an accurate calculation.