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.