A special magnetodynamic effect arising from high-speed testing of conductive objects by magnetic flux leakage method was studied. At a significant speed of motion of the magnetizing system relative to the test object, under the influence of eddy currents, the region of the maximum level of magnetization shifts in the direction opposite to the direction of motion. As a result, a magnetization zone forms behind the back pole of the magnetizing system—a tail field. The structure of the tail field and the possibilities of using the results of its leakage to obtain defect data were investigated. The rail was taken as a test object, since it is precisely when inspecting the railway track that the testing speeds are high and the magnetodynamic effects play an important role. The results of a three-dimensional computer simulation indicate the presence of two oppositely directed magnetic fluxes behind the back pole of the magnetizing system, one of which propagates in the bulk of the metal, and the other on its surface. The resulting distribution of the magnetic field behind the back pole can be used to differentiate signals from surface and internal flaws.