Technical area The present invention concerns a lubricant applicator for a wheel of a trackbound vehicle, which applicator comprises an applicator cylinder for a lubricant stick. The present invention also concerns a method of lubricating a wheel of a trackbound vehicle with the help of a lubricant applicator which comprises an applicator cylinder for a lubricant stick. Technical background A wheel intended for a railway carriage, a railway engine, a railcar, or some other trackbound vehicle, has a tread surface, which is intended for rolling against the upper part of that rail over which the wheel is to roll, and a flange, which is responsible for the guiding against the rail, such that the trackbound vehicle does not leave the rails. The tread surface of a wheel of the above described type rolls, with very low friction, against the upper side of the rail. The flange of a wheel of the type described above will, however, come into sliding contact with the sides of the rail. This sliding contact causes friction and an unwanted wear on the both the wheel and the rail. US 5,054,582 describes a lubricant applicator which is arranged to press a solid lubricant stick against that position on a wheel where a tread surface and a flange are joined in order to decrease the friction described hereinbefore. A problem with the lubricant applicator which is disclosed in US 5,054,582, is that the lubricant stick easily gets stuck in its holder and is not transported forward in the desired manner. Summarv of the invention A purpose of the present invention is to provide a lubricant applicator for application of lubricant on a flange which is comprised in a wheel for a trackbound vehicle, which lubricant applicator requires little maintenance. This purpose is achieved by means of a lubricant applicator for a wheel of a trackbound vehicle, which applicator comprises an applicator cylinder for a lubricant stick, characterized in that the lubricant applicator further com¬prises a piston, which is arranged to run in the applicator cylinder and to press the lubricant stick against said wheel, and a space inside the applicator cylinder which is arranged to receive a pressurized fluid for pressing the piston against the lubricant stick. An advantage of this lubricant applicator is that a reliable lubrication is ensured. A further advantage is that the pressure with which the stick is pressed against the wheel is substantially independent of how worn the stick is. According to a preferred embodiment the applicator cylinder has a cir¬cular cross-section. An advantage of this embodiment is that efficient sealings between the applicator cylinder and lubricant stick can be easily achieved, with the help of, for example, 0-rings. According to a preferred embodiment a control device is arranged to receive at least one signal from a sensor, which senses at least one operating parameter of the trackbound vehicle, and to control the pressure of the pressurized fluid supplied to the space depending on this signal. An advan¬tage of this embodiment is that it becomes possible to apply lubricant depending on the present need for lubricant. In for example curves more lubricant can be applied, and less lubricant may be applied at straight stretches. This reduces the average consumption of lubricant. According to a preferred embodiment both the lubricant stick and the applicator cylinder are formed from electrically conductive material and are in electric contact with each other, wherein currents that pass from a rail to said wheel can be conducted via the lubricant stick and the applicator cylinder further to the trackbound vehicle. An advantage of this embodiment is that the electric load on bearings of the wheels, caused by current being conducted through these, can be reduced, with reduced maintenance costs as a conse¬quence. According to a preferred embodiment the lubricant stick is formed from a material comprising graphite. Graphite has the advantage that it both has lubrication qualities and is also electrically conductive. According to one embodiment a part of the lubricant stick forms that piston which runs in the applicator cylinder. An advantage of this embodiment is that very few parts are required, since the lubricant stick will both work as stick, at its end being directed towards the wheel, and as piston, at its end being directed towards said space. According to another preferred embodiment a separate piston is arranged to run in the applicator cylinder and to be pressed against the lubri¬cant stick. An advantage of this embodiment is that the safety to leakage of pressurized fluid is increased, which is particularly important if pressurized fluids that should not leak into the surroundings, such as hydraulic oil, are used. According to a preferred embodiment the applicator cylinder is releas-ably attached to a bracket which is mounted on the trackbound vehicle. An advantage of this embodiment is that exchange and maintenance of the lubri¬cant stick becomes very neat. A further purpose of the present invention is to provide an efficient and reliable method of applying lubricant to a wheel of a trackbound vehicle. This purpose is achieved by means of a method of lubricating a wheel of a trackbound vehicle with the help of a lubricant applicator which comprises an applicator cylinder for a lubricant stick, wherein the lubricant applicator further comprises a piston, which is arranged to run in the applicator cylinder and to press the lubricant stick against said wheel, and a space inside the applicator cylinder which is arranged to receive a pressurized fluid for press¬ing the piston against the lubricant stick, wherein at least one operating parameter which describes the present operating condition of the trackbound vehicle is measured, and that the pressure in said space is adapted to provide a desired lubrica¬tion of said wheel with regard to said present operating condition. An advantage of this method is that an efficient and controlled lubrica¬tion of the wheel is achieved. Preferably said at least one operating parameter comprises at least one of the present curve position of the trackbound vehicle and the present speed of the trackbound vehicle. The curve position, that is, whether the vehicle is in a curve or not, and how tight such curve is, and the speed are two of those parameters that have the largest influence on the need for lubri¬cation. Further advantages and features of the invention will become apparent from the below description and the claims. Brief description of the drawings The invention will hereafter be described with the help of examples of embodiments and with reference to the enclosed drawings. Fig. 1 is a side view and shows schematically a lubricant applicator for application of lubricant on a flange which is comprised in a wheel of a track-bound vehicle. Fig. 2a is a cross-section and shows a lubricant holder forming part of the lubricant applicator as seen from the side thereof. Fig. 2b is a cross-section and shows the lubricant holder illustrated in fig. 2a as seen in the cross-section lib-lib. Fig. 3 is an enlarged view and illustrates the portion III illustrated in fig.2a. Fig. 4 is a side view, and shows schematically a control system com¬prised in the lubricant applicator. Fig. 5 is a cross-section and shows a lubricant holder according to an alternative embodiment, as seen from the side thereof. Detailed description of preferred embodiments of the invention Fig. 1 illustrates a lubricant applicator 1 in accordance with one embodiment of the present invention. The lubricant applicator 1 has a lubri¬cant holder 2 and a control system 4, both of which are to be described in more detail hereinafter. The lubricant applicator 1 is arranged for pressing a solid lubricant stick 6 against a wheel 8, the outer portions of which are illu¬strated in fig. 1, of a tracl