Mechanical Engineering Question:

That being said railroad track ballast is the cover for the subgrade. Ballast has several functions:

* a) It enables water to drain from the track
* b) It assists in helping control the thermal expansion of continues welded rail
* c) As a train passes the rails, it supports the train
* d) It distributes the weight of the train from the track structure to the sub grade
* e) Maintains a smooth running surface for the train to run on

As ballast becomes contaminated with other materials, it looses its ability to do some or all of these jobs.