This remote sensor is two block detectors on one board which can be mounted remotely from the BDL162 or BDL168. It gets track power, to feed to the detected block, from any track power bus nearby. Its sensing signal is then transmitted to the BDL162 or BDL168 via small signal wires, such as ribbon cables. The purpose is to eliminate the need to run separate track power bus wires from the BDL162 or BDL168 to all reaches of the layout. Transponding is not possible unless the sensed block wire goes through the RX4 that is connected to the BDL162 or BDL168. As such, this makes the RD2 almost useless for this purpose. One example of how transponding can be done with the RD2 is shown at right. What this scheme does is reduce the number of large track power wires going out to the blocks from four to one. While you have eliminated three large track power wires, you have added six smaller [ribbon cable type] wires. We don't recommend using RD2s with transponding when the blocks are close to the BDL162 or BDL168. But if the detection blocks are far from the BDL162 or BDL168, this can be advantageous. This means that a BDL162 or BDL168 may have some blocks wired directly with other blocks wired with RD2s.