This paper presents a mathematical model for rail potential calculation in the DC electrified railway with two traction substations. The rail potential and traction substation voltage are essential for supply equipment, passenger safety and railway infrastructure protection against corrosion, all of which must comply with requirements and standards. The classical and simplified mathematical models are described in this study to analyses the rail potential and current. In addition, the simplified equations for estimating the rail potential and traction substation voltage are derived to simply determine the stray current. These equations can be used to analyses and calculate the stray current based on the approximate rail potential. They are also validated according to the equations in EN 50122 2.