What is the role of the subgrade layer?

The role of the subgrade layer is to provide a stable and uniformly supportive foundation for the entire track structure, including the ballast, ties, and rails, and to distribute the load from the train to the underlying soil. The subgrade is the native soil or engineered fill that is located beneath the ballast and other track components. Its primary function is to bear the load imposed by trains and transfer it to the underlying ground. A well-constructed subgrade will be properly compacted and graded to provide a smooth and level surface for the ballast. It should also be stable and resistant to deformation under load. The subgrade must also provide adequate drainage to prevent water from accumulating beneath the track structure, which can weaken the soil and lead to track instability. In some cases, a subballast layer is placed between the subgrade and the ballast to provide additional drainage and filtration. The subballast is typically a layer of gravel or crushed stone that helps to prevent fine soil particles from migrating into the ballast, which can impede drainage and reduce its effectiveness. For example, if the subgrade is composed of soft clay, it will be prone to deformation under load, leading to track settlement and instability. In this case, the subgrade may need to be stabilized by adding a layer of compacted gravel or by using chemical stabilization techniques. The subgrade is the foundation, so if it fails, then the track structure is prone to failing as well.