Railway sleepers have important roles in the complex railway system. Throughout therninternational railway community, many methods have been developed to design and analyzernthe concrete sleepers. Specifically, this paper focuses on how changes in support conditionrnaffect the internal behavior of the sleeper. Due to different loading condition, poor maintenancernor bad quality of ballast, a random load distribution along the sleeper-ballast interface mayrnoccur. A sleeper design, and also the track system design, which do not consider the randomrnload distribution, could influence the performance of the sleeper and even damage the wholernrailway system. Thus, a numerical analysis for a pre-stressed concrete mono-block railwayrnsleeper is carried out using finite element method. The internal behavior of a single sleeperrnsubjected to a varying sleeper-ballast interaction is studied and, a linear-elastic sleeper analysisrnmodel was developed. This model was used to perform a study to determine the sensitivity ofrnthe sleeper to changes in ballast reaction along the sleeper. Four different possible scenarios ofrnsupport conditions for a sleeper with varying contact length are discussed in numerical analysis.rnResults of vertical displacement and flexural stress of both rail seat and at mid-section ofrnsleeper are presented. According to the simulations conducted in this study, the bendingrnmoments due to the applied rail seat load at the sleeper rail seat / center can be exceededrnfrom design bending moments under small variation in distribution of the ballast reaction.rnBesides very soft support underneath the rail seat and large variation of ballast stiffnessrnalong the length of sleeper might cause severe cracking problems especially at the toprnsurface in the middle of the sleeper. The worst support condition is also identified.rnMoreover, the results of this study were compared to existing design recommendationsrn(Australian Standard AS 1085.14) to find allowable levels of ballast reaction that can occurrnbeneath the sleeper before failure is expected.