AC supply and DC supply coexist in electrified railway in some countries, such as South Africa. When a locomotive runs to a railway section powered by DC, its transformer is usually used as a filter reactor. In this paper, the electromagnetic characteristics of a locomotive transformer in DC system were studied with a geometry of "C-core" core-type and four high voltage windings and four low voltage windings symmetrically arranged on two core limbs respectively. When a locomotive is powered by DC, the four high voltage windings of a transformer are connected in parallel and in open circuit, and the two low voltage windings on a limb are connected in series and used as the filter reactor. There are two DC powered circuits in a locotomotive and one of them can work alone or the two work together. In this paper, the magnetic field of the transformer was analyzed and the inductances were calculated, in which different connections of the four low voltage windings were considered. Moreover the characteristics of inductances with changes of the currents in the windings were studied, and the inductance computation was proved with tests for that in which a DC circuit works alone. Then, a proper connection of the low voltage windings in DC system was proposed according to the above analysis. Finally, some investigation was done to increase the inductance when a DC circuit works alone and an improved design was presented.