Direct-current motors with series field windings are the oldest type of traction motors. These provide a speed-torque characteristic useful for propulsion, providing high torque at lower speeds for acceleration of the vehicle, and declining torque as speed increases. By arranging the field winding with multiple taps, the speed characteristic can be varied, allowing relatively smooth operator control of acceleration. A further measure of control is provided by using pairs of motors on a vehicle in series-parallel control; for slow operation or heavy loads, two motors can be run in series off the direct-current supply. Where higher speed is desired, these motors can be operated in parallel, making a higher voltage available at each and so allowing higher speeds. Parts of a rail system might use different voltages, with higher voltages in long runs between stations and lower voltage near stations where only slower operation is needed.