The rheostat (connected in series to the traction motors) is necessary because the DC motor has the intrinsic characteristic of absorbing a current inversely proportional to its rotating speed; at high speeds, it absorbs less current. This means that on start the current would be very high, because the only resistance encountered would be only the one offered by motors and internal conductors, which is very low (a short circuit, in practice). The rheostat increases the overall resistance when starting the locomotive, lowering the current and allowing a smoother start.