The temperature change in continuously welded rail (CWR) tracks induces substantial internal stresses and shifts sleeper positions because the rails are longitudinally restrained from thermal expansion. This phenomenon is significantly intensified at tunnel portals, where a sharp contrast exists between the thermal boundary conditions of the open track and those of the sheltered tunnel environment. The current article investigates the dynamic interplay between rail dilatation and axial forces at these critical junctions by employing a finite-element (FE) model of the 54E1 rail track, calibrated using experimental measurements of track fastening parameters and ballast behavior. The research specifically examines the combined influence of temperature gradients between the tunnel and open environments, the bilinear longitudinal resistance of the ballast, and the mechanical braking loads exerted by passing trains. Through a series of parametric studies, the results demonstrate that simultaneous thermal and braking forces can trigger extreme rail displacements of up to 100 mm and axial forces of up to 1.4 MN. Notably, such high-stress states occur even when the ballast resistance is only 7 N/mm lower than the braking force. While increasing track fastening resistance helps equalize the impact of braking and thermal effects, it effectively reduces deflections to non-critical levels. The most severe stability risks are identified when the center of the braking zone aligns precisely with the portal. Ultimately, the study concludes that ballast resistance is the decisive factor in managing track integrity at tunnel entrances.