Dynamic amplification effects may occur when a railway line, with high operation speed, is in a soft soil. In these scenarios, it is necessary to analyse the impact the critical speed phenomenon on safety and operational costs of the railway track. A ground treatment may contribute to an improvement of the system dynamic behaviour. This treatment may be based on replacing a portion of the natural soil with stiffer materials or by employing grouting. The current paper presents a numerical study that shows the improvement of the dynamic behaviour of the ground-track system obtained by means of the soil reinforcement. This study also evidences that localized ground reinforcement below the track yields a response similar to extending the reinforcement over a larger area. This conclusion is particularly interesting for engineering practice and for numerical/semi-analytical analysis. The definition of the critical speed as the value that induces larger amplification due to the passage of a single load is a conservative approach. Finally, it was possible to confirm that an approach based on the calculation of the dispersion curves provides an adequate estimation of the system’s critical speed.