This work shows transmission parameters of specific power line, which is control cable used for supplying a railway sign. Two transmission parameters are considered, namely: attenuation and characteristic impedance, these two secondary parameters are calculated from primary parameters i.e. R, L and C. Primary parameters and their components were calculated if they depended on the frequency or were taken from producers notes. Analysis was done for the frequency in range from 50 kHz up to 100 kHz. Two types of cables were analysed, as extreme cases, first with the 1 mm2 solid conductors cable and second with the 1.5 mm2 19 strands conductors. Knowing the PLC modem transmission parameters together with the transmission parameters of the railway sign power line it is possible to calculate the capabilities of the communication system e. g. distance or maximum bit rate.