This KA4 Series Keep-Alive(R) device from TCS is used to supply power to decoders during times of power interruption due to dirty track or problematic track work. The KA4 can power a locomotive for 1-2 seconds over dirty track and power interruptions (depending on the number of lights and efficiency of the motor). The result is that if your locomotive loses electrical contact over a long switch frog or a short stretch of dirty track, it'll keep on running as though the problem didn't exist. KA4 series Keep-Alive units are sized to fit HO and larger locos. Note: for N Scale and small HO locos check out the KA-N1 Keep-Alive.