Concrete ties failed due to "thud".

Versión en español aquí: http://ingjoseguerrero.blogspot.mx/2013/09/durmientes-de-concreto-fallados-debido_19.html

In the Mexican railroad slang, the term "thud" refers to an area of the track where the ballast is not properly tamped and you can see, when the train is passing over, that its level suffers failures. In times where the train is not passing it seems that the area is properly leveled, with reference to the whole track.
Areas with "thud" causes ties work as a beam without support.

The concrete ties, like any other concrete element, are susceptible to failure when subjected to loads that are not capable of being transmitted to the supports.

In the following pictures we can see concrete ties that have failed due to a bad tamped ballast beneath them, and they are on areas with "thud".

Picture # 1: Concrete tie with RNY fixing. (click to enlarge)

Picture # 2: Concrete tie with Pandrol fixing. (click to enlarge)