CLEARSY develops a range of safety relays for interfacing with railway systems, including on-board systems. These relays interface with SIL2, SIL3 and SIL4 safety PLCs according to EN50129, and some are intrinsically safe and perform their own contact monitoring. RS4 relays are certified, similar in function to the SIL4 intrinsically safe NS1 signal relays. Safety relays are also used in relay stations.