This study provides a simulation evaluation of the effect of fouling conditions on the thermal behavior of ballast for the use of railway tracks. Ballast fouling can result in a slurry pumped up to the surface, causing poor foundation strength, rotting of the ties, and other ill effects. To achieve a quick and convenient detection of ballast fouling, a thermal-based non-contacting technique has been proposed and becomes more and more attractive. However, the successful application of this thermal-based fouling-detection technology requires knowledge of the thermal characteristics of ballast, which have not been investigated in prior studies. The objective of this paper is to study the influence of fouling on the thermal behavior of ballast, using an analytical model developed based on one-dimensional conductive heat transfer. The effort to validate the developed model is also included. The general fouling conditions of the ballast — fouled with and without water — are studied through simulation. The simulation results show that, for the case of fouling without water, the ballast under different fouling conditions behaves differently from clean ballast at depths ranging from 0 to 12in under naturally-occurring daily ambient temperature changes, and that the temperature difference peaks at 4-in depth. In addition, increasing the amount of fouling results in less temperature variation in response to the ambient temperature changes. For the case of fouling with water, water is added into the 100% fouled ballast and comparisons are made between the ballasts with different water content. A similar pattern is observed, showing that increasing the amount of water results in a larger temperature difference at all depths considered. Moreover, the maximum temperature difference is observed at the top surface rather than the depth of 4-in as observed in the case of fouling without water.