The performance of ballast is a critical element for railway track structural stability. Open-graded ballast supports and surrounds railway ties, creating an open track structure subject to water infiltration. Ballast can provide good drainage, but ballast deterioration may limit drainage capacity and impact track performance and stability. Track instability problems are discussed to highlight the importance of ballast and introduce the language currently used in track safety statistics to describe these problems. Track safety statistics will be reviewed to highlight some of the problems and the challenge of describing the mechanics of these failures during accident investigations. Then, impacts of ballast on the structural stability of track will be reviewed. Conditions indicative of the identified structural performance limitations that can be imposed by poor ballast condition will be highlighted along with potential methods to measure or otherwise monitor the influence of these conditions on track support and safety.