Ballasted tracks are very common in many countries including Sri Lanka due to its low capital cost, high resiliency and high drainage capacity. However, progressive contamination of ballast by plastic fines due to soft subgrade pumping leads to decrease its drainage capacity. In order to study the effect of plastics fines on the drainage capacity of the track, a numerical analysis was conducted using SEEP/W to quantify the drainage capacity of ballast under different levels of contamination. Actual track geometry was simulated and the experimentally derived permeability values were used as input parameters. In this study, the drainage classification chart is proposed based on the maximum permissible rate of rain fall. The track drainage values obtained from this numerical model were then analysed in order to determine the drainage category based on the classification chart.