###### Abstract  
The criteria for selecting materials for railway pavement (particle size distribution, California Support Index, and physical indexes) are not suitable for evaluating the hydraulic behavior of tropical soils under unsaturated conditions, often resulting in the rejection of soils with good mechanical and hydraulic performance. The railway tracked is exposed to precipitation, allowing water infiltration into the pavement layers and consequently leading to moisture variation. Therefore, the present study aims to assess the moisture variation over time in the railway tracked when subjected to the action of rainfall. For this purpose, five soils were analyzed as constituent materials of the subballast, with the infrastructure of the Carajas Railway in Brazil being used as a reference. Water retention curves and conductivity tests were conducted using the HYPROP (Hydraulic Property Analyzer) and WP4-C (Water Potential Four) equipment, and numerical modeling was developed using the IVFlow software. The numerical modeling revealed that sample 1 (latertic sand) performed better compared to the other analyzed soils, as it exhibited lower moisture variation (3.25% above the optimum content) under the influence of rainfall and it also had low permeability. railway track; unsaturated; soil water retention curve +
Footnote †: journal: Applied sciences  
Maintenance-of-Way Association (AREMA) [3], which govern railway pavement design, do not offer specific criteria for this type of material. The Carajas region in Brazil is known for its abundance of lateritic soils, which are frequently used in railway construction. However, the behavior of these soils under variable moisture conditions is not fully understood. Unlike the stone materials used in international railways, lateritic soil is susceptible to water impact, and infiltration can compromise the railway platform, leading to a reduction in resilient modulus and an increase in permanent deformations. When it comes to the characteristics and main functions of the subballast layer, there is some discordance among authors regarding the functions that it should fulfill, as per Figure 1.  
[PERSON] [5] and [PERSON] [6] proposed that the subballast layer should enhance the bed's erosion resistance and impede water penetration, thereby maintaining the subgrade's properties. In contrast, [PERSON] and [PERSON] [7] proposed that the subballast layer should serve as a drainage layer to facilitate the removal of water that rises through the subgrade by capillarity. Furthermore, it is stated that the subballast must adhere to the Terzaghi filter criterion by possessing a certain level of permeability that enables drainage, while also preventing the migration of fines into the underlying layers. In contrast, [PERSON] et al. [2] suggest that the subballast should be impermeable to safeguard the subgrade from erosion. However, in agreement with [PERSON] and [PERSON] [7], [PERSON] et al. [1] assessed that the layer should be permeable enough to enable water to flow and channel it towards the lateral drainage devices. It should also permit the rise of water from the subgrade, alleviating excessive pore pressures when saturated. To achieve this, the subballast must meet the Terzaghi filter criterion [8]. AREMA [3] recommends that the subballast layer should be both impermeable to prevent saturation of the subgrade and permeable enough to allow for a capillary rise of water, which can help prevent saturation. However, it is worth noting that the authors who advocate for the subballast layer to adhere to the Terzaghi filter criterion have focused on studying stony materials. Given that this study employs tropical soils as a subballast material, it is important to acknowledge that, as pointed out by [PERSON] and [PERSON] [9], lateritic soils classified as tropical soil have promising potential for pavement use due to their high resistance and low permeability, qualities of great interest in permanent way projects.