A particular concern in ground-borne vibration is the extent to which the construction of a foundation affects the vibration field from an existing railway tunnel. This paper presents the results of a parametric study of this ‘added-pile effect’ using a numerical model that captures the dynamic behaviour of a single pile located near a circular tunnel, excited by vibration due to wheel-rail roughness. The added-pile effect is characterised for a range of scenarios in terms of the vertical pile-head insertion gain over the frequency range most associated with human perception. The results highlight two underlying mechanisms that determine the pile-head response and how these are governed primarily by the pile length, the soil-pile stiffness ratio, and the relative position of the pile and tunnel. It is also shown that the addition of a representative building, in the form of a semi-infinite column, does not significantly change the pile-head response.