A switched-mode unit used in electric locomotive generates a strong high frequency conducted electromagnetic interference (EMI),which radiates electromagnetic energy through railway lines.Evaluation of magnetic field using analytical technique based on contour integral is presented,in order to assess the electromagnetic environment around a high-speed railway.Actual railway multiconductor finitely long overhead lines are represented by an infinitely long single line above two-layered earth,whose characteristic is different from homogeneous earth.Owing to the constraint of the GB/T 24338-2009 and the high frequency investigated (a few MHz),only the magnetic fields are examined.The magnetic fields consist of four components:the direct wave,the ideal reflected wave or image wave,the trapped surface wave,and the lateral wave.The calculation results proved that due to the presence of the trapped surface wave,the magnetic field of the observer point on the interface is strongly influenced,when the line is on or closed to the interface. A switched-mode unit used in electric locomotive generates a strong high frequency conducted electromagnetic interference (EMI), which radiates electromagnetic energy through railway lines. Evaluation of magnetic field using analytical technique based on contour integral is presented, in order to assess the electromagnetic environment around a high-speed railway. Actual railway multiconductor finitely long overhead lines are represented by an infinitely long overhead line are two-layered earth, whose characteristic is different from homogeneous earth. Outwing to the constraint of the GB / T 24338-2009 and the high frequency investigated (a few MHz), only the magnetic fields are of four components: the direct wave, the ideal reflected wave or image wave, the trapped surface wave, and the lateral wave. that due to the presence of the trapped surface wave, the magnetic field of the observer point on the interface is strongly influenced, wh en the line is on or closed to the interface.