Answer:
The center of the rail rise above the ground is:
15 meter.
Explanation:
As the railroad on expansion will form a right isosceles triangle.
The railroad is 3 km=3000 m before expansion.
It expanded by 15 cm.
This means that in meters it expanded by 0.15 m
Hence, the railroad after expansion it is: 3000.15 m
Now we need to calculate the hypotenuse of the right triangle in order to find the center of the rail rise above the ground.
and hence it is calculated as:

which in meters is approximately given by:
15 meters.