What is the purpose of ballast in railroad construction?

The purpose of ballast in railroad construction is to hold the track in line and surface using coarse gravel. Ballast provides essential support for the weight of trains and helps maintain the stability and alignment of the tracks. It acts as a foundation, allowing the tracks to withstand the forces applied by moving trains while also facilitating proper drainage to prevent water accumulation that could undermine the track structure. By providing this stability, ballast plays a crucial role in ensuring safe and efficient train operations. The other options do not accurately reflect the primary function of ballast in the context of railroad construction.