Inductive interference effects in the Hong Kong Island Line signalling system

This paper describes inductive interference effects, which exist when track based cables are used for track-to-train speed commands in a railway system, which also uses jointless track circuits for train detection. It is shown that spurious speed commands (ghost signals) can be transmitted to trains not intended to receive the original message, and that in some circumstances this could be dangerous. The paper is based upon theoretical and practical investigation undertaken for the new signalling system for the Island Line extension of the Hong Kong Mass Transit Railway. The paper demonstrates that providing appropriate design criteria are adopted, it is practicable to introduce jointless track circuits, together with tightly coupled track-based conductor loops, to a railway which originally used jointed low-frequency track circuits for train detection, together with track-to-train speed commands using the rails as a transmission path.