Sweden has a very high and variable soil resistivity, making it difficult to ensure a consistently good connection to earth along the track. Booster Transformers (BTs) have been used to ensure that the current returns through the intended path and that stray currents are limited. The ability of BTs to control return currents is limited by their series impedance and by imperfect coupling. In this article, we make a detailed model of the BT system between two feed-in points and evaluate how well the BT system can contain stray currents at harmonic frequencies. The main contribution is that we demonstrate that harmonic currents are significantly less well contained by the BT system, and that the practice of allowing local grid connections to the railway earth system risks creating significant stray currents in the local grid, particularly at harmonic frequencies, but also that electrical safety may be compromised by the transmission of touch voltages to locations with a different soil resistivity than the rail bed.
Hjertberg et al. (Sat,) studied this question.