The aim of this research is to find out how the common earth network affects the level of the traction return current in the $2{ imes}25 kV$ Power Supply System. The traction return current plays a significant role in the operation of the facilities near the rails. It is shown that the common earth network in the $2{ imes}25 kV$ power supply system not only minimizes the level of the traction return current, but also increases the safety of the working crew on the railways. In order to determine the relationship between common earth network and the traction return current, we investigated the earth system of the Gyongbu High Speed Line that is constructed following the SNCF regulations. We carried out the field test in the Osong station. The results of the test show that the common earth network minimizes the effect of the traction return current. We also find that the simulated results are very similar to the test results. We concluded that the results of the test can be applied for the rest of the Gyongbu High Speed Line under construction.