The article describes a project to use PLC technology in railway light signs. The proposal presents a solution for the separation of supplying circuits with the use of filters. The necessity of separation is a result of long distance data transmission between sign controllers and LED railway signs using PLC modems which work on the same frequency. Problems in the selection of filter parameters are presented due to the fact that there is no specification for the characteristic of the separation filter, there are only requirements for the reliability communication system. To meet these requirements, the circuits separation efficiency must be assessed. The paper presents a quick method to evaluate separation efficiency. Structure of the network, as well as the set of devices which realize this specific kind of wired sensor network for supervising railway LED sign network and maintenance parameters are also presented in this article.