We invite you to visit this link to our ZeticaRail UK group's site.
There are some very good examples of the importance of regular trackbed monitoring. There are references to Irish Rail, Union Pacific, BNSF, Network Rail and Etihad Rail
The importance of timely intervention is constantly reinforced by analysis of these and other case studies. Dependence on traditional trackbed inspection strategies such as track patrols, track geometry measurements, etc, are generally a 'call to action' at the later stage of subgrade and ballast quality related issues.