This paper attempts to provide a better understanding of the service quality risks that are associated with freight train operations in mixed traffic. Adequate operational quality requires a certain level of robustness to delays and disruptions, but on segments with confined capacity, robustness becomes more fragile with increased traffic demand and speed heterogeneity. This trade-off between capacity utilization and service robustness predominantly manifests itself by compromising the reliability of rail freight services for two main reasons. Firstly, passenger trains are often prioritized in dispatching over freight trains. Secondly, many freight trains operate over longer distances and they accumulate more delays. Decreasing speed heterogeneity in mixed traffic may become pivotal in the interplay between robustness and capacity utilization. In this paper, we investigate possible improvements in capacity utilization and in railway service quality when introducing faster freight trains in mixed-traffic operations. The analysis is carried out on the Swedish Southern Main Line, which forms part of the Scandinavian-Mediterranean freight corridor. Microscopic simulation is used to explore performance indicators, such as simulated running times with respect to scheduled running times, capacity utilization, and punctuality, by implementing and comparing scenarios with different maximum speeds for freight trains. The results suggest that increasing the top speed of freight trains might seem as a promising approach for reduced utilization of capacity in the planning stage, but the stochasticity of operations in mixed traffic may become more challenging for delay recovery.