Modern track is often continuously welded, the joints being welded during installation. This offers many benefits to all but the signalling system, which no longer has natural breaks in the rail to form the block sections. The only method to form discrete blocks in this scenario is to use different audio frequencies (AF) in each block section. To prevent the audio signal from one section passing into an adjacent section, pairs of simple tuned circuits are connected across the rails at the section boundary. The tuned circuit often incorporates the circuit to either apply the transmitted signal to the track or recover the received signal from the other end of the section.