Upgrading existing passenger-only railway lines to carry freight traffic is becoming increasingly desirable. This is challenging because freight trains have larger axle loads and thus can have a negative effect on track longevity, particularly on ballasted lines supported by sub-optimal ground conditions. These additional loads can cause large subgrade strains resulting in non-linear behaviour, which should be considered before permitting freight vehicles on passenger routes. To do so requires the modelling of non-linear soil behaviour which is challenging. Therefore, this paper presents a solution in the form of an equivalent non-linear, thin layer element soil model, coupled to an analytical track model. The model has low computational demand and can adjust subgrade stiffness depending upon strain levels. Therefore, it is well suited to computing track response induced by freight trains. This paper validates the model and then uses it to compare the differences between the response of a ballasted line to freight and passenger vehicles.